Japan expecting to end WWII by treaty was one of those foundation assumptions no one questioned, after all that is how all of Japan's modern wars had ended, also WWI, and as of 1941 no one believed the US could mount an invasion of Japan, blockade it yes, invade no. In 1945 the US trans continental rail links could not carry the necessary traffic, ships had to be loaded in the east coast ports, to give an idea of the invasion effort required.
On to 1940 Britain. Keith Park was worried about the fighter control system, not airfields, given most were large grass fields knocking them out was hard, the control rooms were on the airfields and above ground, the Germans used direction finding to figure out where to bomb. The fighting in late August and early September was slowly grinding the RAF down but the Germans were on a strict timetable. The strikes on London did what the Luftwaffe hoped, draw in most to all available RAF fighters, making it possible to really up the RAF loss rate.
Given a lack of German naval power and little ability to land heavy weapons, the Luftwaffe was required to defeat and keep defeated Fighter Command, do pre invasion softening up (some hopefully as part of the anti Fighter Command operation). Then from invasion day add, escorting paratroop operations, shield the landing zones and shipping from the RAF and RN, including anti shipping and raids on airfields, provide battlefield support.
A big part of amphibious warfare is the follow up, continually landing more troops and supplies. Getting only one wave ashore probably means defeat. A battleship and even a cruiser are not much better at sinking much the sort of invasion shipping used by the Germans than destroyers, so how many the RN would risk is an open question.
In June 1940 Milch put a plan to Goering to use the remaining paratroopers to create an airhead in Britain, take an airfield and then fly in reinforcements.
From a discussion about Sealion, mostly not my contribution
British Army in Britain, 548 Cruiser Tanks had been built by 30 September 1940; 134 had been lost in France and 137 were in Egypt, which still left over 200 in Britain, enough to equip the four Cruiser regiments of 1st Armoured Division that were active. The situation with Infantry Tanks was even brighter, of 524 completed, 100 were lost in France and 52 had been shipped to Egypt on 18 August leaving well over 350 available, enough to equip the five regiments of the GHQ Reserve 1st and 21st Army Tank Brigade to full War Establishment and leave enough left over to allow the continued training and equipping of the six regiments of 23rd and 24th Army Tank Brigades (essentially a full regiments worth per brigade). And there were a plethora of Light Tanks available, of 1,340 built, 321 had been lost in France and 275 were in Egypt, leaving over 700, more than enough in theory to equip the dozen or so regiments formed to War Establishment. Note that by actual count the three 'light' regiments of 22nd Armoured Brigade averaged 42.33 tanks, 73 percent of their War Establishment of 58. Then, from October to December 1940 another 449 tanks were built, 78 cruiser tanks, 354 'I' tanks, and 17 light tanks.
In terms of newly produced artillery, not counting guns evacuated from France (a total of 322 of all types or roughly one-in-eight had been brought back), there were about 140 2-pdr AT guns, 568 40mm Bofors AA guns, 294 25-pdr guns, 728 3.7-inch AA, and 118 other miscellaneous guns added to the Army from 1 April to 30 September. Also by about 1 October 1940 most, if not all of the shipments of equipment purchased from the US on 11 June had arrived, including 895 M1917 and M1897 75mm guns, each with 1,200 rounds of ammunition, 300 3-inch mortars, each with 325 rounds of ammunition, 1,157 Lewis MG, 7,071 Vickers MG, 10,000 M1917 MG, and 25,000 BAR.
In addition, large numbers of reserve stocks had not been sent to France and so were available. The numbers of artillery in the UK as of the end of May 1940 were: 126 18-pdr, 269 18/25-pdr, 90 25-pdr +252 produced June-September = 342. So 737 total field guns, sufficient for 30 regiments with a small reserve of 17 pieces. That's enough to fully equip 10 divisions. Now a quick check shows there were just 17 regular regiments in the UK at that time, plus 5 RHA, while there were 82 TA, so 104 regiments to be equipped and a requirement for 2,496 pieces in total. That of course amounts a minimal shortfall of about 864 guns to WE, even if we add in the 895 US guns.
Medium guns, 321 4.5-inch howitzer, 14 60-pdr guns (being converted to 4.5-inch), 5 4.5-inch/60-pdr guns (converted), 94 6-inch howitzer. There were something like 27 medium regiments formed, so a requirement for 432 pieces. The 434 available were just sufficient, especially when the production of June-September is added in.
Heavy guns, 20 6-inch gun, 14 8-inch howitzer, 39 9.2-inch howitzer (siege), 29 12-inch howitzer (railway and siege), there were some 12 heavy regiments to man these pieces and to supplement personnel manning the coast defenses.
In terms of 'B' Echelon vehicles, 63,879 had been lost in France, but 54,057 new ones had been produced. The shortfalls there were made up by requisitioning public transport and private motor vehicles.
All US types were utilized at some point by British regular and territorial Home Forces until they could be replaced by regular equipment, being then transferred successively to the Home Counties Brigade Groups (later divisions), then the Home Guards, and finally also to foreign areas and allies (some were later shipped to British Forces Middle East as antitank weapons and then to equip Free French, Greek, and other forces in Egypt and Tunisia). As of mid-September to late-October 1940 (arguably the endpoint of the Sealion 'threat') the following US weapons deployment at a minimum have been identified.
45 (Wessex) Division (2nd Line Terr.) - sector Dymchuch (excl.)-Telscombe 55 Field Regiment (14 - 75mm, two sited as AT, 6 4.5-inch how, 4 -25-pounder) 142 Field Regiment (10 - 75 mm, two sited as AT, 8 - 4.5-inch how) 96 Field Regiment (12 - 75 mm guns, 6 - 4.5-inch hows, 4 - 25-pounder, 2 - 6-pounder sited as AT)
New Zealand 'Division' - in XII Corps reserve 5th NZ Field Regiment (E and F Battery - 8 75mm (ex-US M1897) each, G Battery - 8 25-pdr Mk II)
Emplaced as 'Emergency' beach defense batteries: Gravesend - East Tilbury Battery - one 75mm Pevensey Battery - two 4.7-inch and two 75mm as AT defense Toll Point Battery - two 75mm
For the other divisions active in defending the coast, expect the numbers to be similar.
'Pre-war' artillery was an important part of the early British artillery forces in France and then in the defence of England in late 40 and early 41. The 18-pounder was critical in supplying carriages in 1939-1940 when 25-pounder gun tube production outstripped 25-pounder carriage production and as a supplement to the relatively small numbers of 25-pdrs available. A total of 611 18/25-pdr were converted 1937-1939 and a further 811 1940-1941 when 25-pounder carriage production caught up with demand. However, in addition, 216 complete 18-pounder were taken to France by the BEF and lost there.
As above, we can find that 1st, 3rd, 8th, and 11th Canadian Field Regiments were equipped with a mix of 25-pdr and 18/25-pdr. 31 Infantry Brigade Group (attached to 45 Division) 75 Field Regiment RA had 24 25-pounder and there were some in the divisional regiments and with the New Zealanders, but the Australians had a mix of 5 Fld Bty (12 25-pdr) and 6 Fld Bty (3 18-pdr and 3 4.5-inch howitzers).
The status of the KM as of mid-September:
Heavy Units
BB Bismarck - commissioned 24 August and on sea trials 14 September-5 December 1940. From 6 December 1940 to 24 January 1941 she is at Hamburg refitting and undergoing final construction changes.
BC Gneisenau - 20 June 1940, torpedoed by HMS Clyde, she is at Kiel under repair from 28 July to December 1940.
BC Scharnhorst - 8 June 1940, torpedoed by HMS Acasta, she is at Kiel for repairs from June to December 1940.
CA Scheer - leaves dock at Wilhelmshaven on 31 July 1940 following a major rebuild of her bridge, sea trials and working up continue until 23 October until she sails from Gotenhafen for the Denmark Strait.
CA Lutzow - 11 April 1940, torpedoed by HMS Spearfish and heavily damaged, losing both props and rudder, she is in repair at Kiel and then working up until 12 June 1941 when she is again damaged, this time by a British torpedo bomber and is in repair until January 1942.
CA Hipper - after an unsuccessful Arctic cruise (25 July-9 August) she puts into Wilhelmshaven for maintenance (12 August-9 September), but while running sea trials on 30 September 1940 she suffers a major engine casualty, which puts her out of action until 28 October.
CA Blücher - sunk Oslo Fjord 9 April 1940.
Number Available = Zero
Light Units
CL Emden - built in 1921 and considered obsolete, used only as a training ship, although she was also utilized as a troop transport for Weserübung. Armed with 8 15cm, 2 8.8cm, and 2 2cm guns, 4 torpedo tubes, and 120 mines.
CL Königsberg - sunk 10 April 1940.
CL Karlsruhe - sunk 9 April 1940
CL Köln - the sole survivor of the K-class, they were collectively considered to be of short endurance and structurally unsound. Armed with 9 15cm, 6 8.8cm, and 8 2cm guns, 12 torpedo tubes, and 120 mines.
CL Leipzig - damaged by torpedoes of HMS Salomon 15 December 1939 and out of commission until 1 December 1940.
CL Nürnberg - armed with 9 15cm, 8 8.8cm. 8 3.7cm, and 4 2cm guns, 12 torpedo tubes, and 120 mines.
Number Available = Three
Training Ship Bremse - armed with 4 12.7cm, 4 3.7cm, and 2 2cm guns.
Destroyers and Torpedo Boats
Z1-Z3 - sunk
Z4 - Available
Z5 - Available
Z6 - Available
Z7 - 25 August 1940 puts into Kiel following major engine casualty. Unavailable.
Z8 - Available
Z9 - sunk.
Z10 - Available
Z11-Z13 - sunk
Z14 - Available
Z15 - Available
Z16 - Available
Z17-Z19 - sunk
Z20 - Available
Z21-Z22 - sunk
Z23 - commissioned 15 September 1940 and working up until March 1941
Z24-Z25 - not commissioned yet.
Z26 - commissioned 11 January 1940 and possibly available.
Number Available or Possibly Available = 10
Torpedo boats
TB Möwe - torpedoed by HMS Taku on 9 May 1940 and in repair until spring 1943.
TB Seeadler - Available
TB Albatros - sunk by coastal batteries at Oslo Fjord 10 April 1940.
TB Greif - Available
TB Kondor - Available
TB Falke - Available
TB Wolfe - Available
TB Iltis - Available
TB Luchs - torpedoed and sunk 26 July 1940 by HMS Clyde.
TB Tiger - sunk in collision 25 August 1939.
TB Jaguar - Available
TB Leopard - sunk in collision 30 April 1940.
T1 - damaged by bomb hit 18 September 1940 and in repair until 5 October 1940.
T2 - Available
T3 - Heavily damaged in air attack at Le Havre 19 September 1940 and not re-commissioned until 12 December 1943.
T4 - commissioned 27 May 1940 and first operational 5 October 1940.
T5 - Available
T6 - Available
T7 - Available
T8 - Available
T9 - commissioned 4 July 1940 and first operational 5 October 1940.
T10 - commissioned 6 August 1940 and first operational 27 October 1940.
T11 - Available, but badly damaged by bombs on 17 September
T12 - commissioned 3 July 1940 and first operational 27 October 1940.
Captured
TB Panther - Norwegian, available, but used for coastal escort between Norway and Germany
TB Löwe - Norwegian, available, but used for coastal escort between Norway and Germany
TB Leopard - Norwegian, available, but used for coastal escort between Norway and Germany
TB Tiger - Norwegian, available, but used for coastal escort between Norway and Germany
TB Troll - Norwegian, too old for active duty and converted to a supply ship by fall 1940
TB Zick - Norwegian, available, but used as harbor patrol vessel in Norway
TB Zack - Norwegian, available, but used as harbor patrol vessel in Norway
KT1 - Norwegian, used as coastal patrol in Norway
NS28 - Norwegian, used as coastal patrol in Norway
Kürassier - Norwegian, used as coastal patrol in Norway
Tarantel - Norwegian, used as coastal patrol in Norway
Balte - Norwegian, used as coastal patrol in Norway, salvaged after being scuttled it was in poor condition
Admiral Deinhard - Norwegian, used as a utility harbor boat
Schlange - Norwegian, used as coastal patrol in Norway
Eidechse - Norwegian, used as coastal patrol in Norway
Schildkröte - Norwegian, built in 1899 and too old for service, used for harbor patrol in Norway
Seestern - Norwegian, built in 1900 and too old for service, used for harbor patrol in Norway
Qualle - Norwegian, built in 1902 and too old for service, used for harbor patrol in Norway
Krokodil - Norwegian, built in 1902 and too old for service, used for harbor patrol in Norway
Total Available = 12. Possibly or Nearly Available = 6 Available, but operating in Norwegian waters = 19 (only four of which may have been useful)
U-Boot
At the beginning of August there were 18 U-Boot available and two new boats became operational. However, two were lost and two old boats were retired as training vessels, leaving 16 available at the beginning of September. Another three became operational in September giving a maximum of 19 available (and the strength did not change in October, one joined but one was lost, leaving 19 - see Clay Blair's two-volume history for additional details on the proposed operations). Total Available = 19, the total fleet was 51 strong end August, including those working up and training types, the start of war strength was 56, the low point 45 in April 1940. If all U-boats are assigned to the invasion it cuts the chances of stopping the US weapons arriving and frees RN anti submarine forces. If the idea is assign all the fleet then they have to be withdrawn from the trade routes in mid August at the latest to be ready. Also both sides were mining the channel, British mining had stopped U-boats using the route in 1939.
Support Vessels To provide fire support for the landings the Germans planned on using a number of ancient, captured vessels. They were:
Norwegian - Harald Harfarge (1897) and Tordenskjöld (1897), each 2 21cm, 6 12cm, 6 7.6cm, and 6 1-pdr guns, their machinery was still capable of 14 kt. They were converted to AA batteries and renamed Nymphe and Thetis.
Holland - Vlieereede (1902) and Ijmuiden (1906), one with one and one with two 24cm guns
Denmark - Niels Juel (1918, 10 15 cm and 15.9 kts)) and Peder Skram (1910, 2 24cm and 4 15cm)
The actual Seelöwe decision making timeline was:
27 August - Hitler decides on the "Small Solution", i.e., a landing between Folkestone and Eastbourne (so about a 50 mile front) by 16. Armee and 9. Armee. The landing by 6. Armee is relegated to being a 'possible' follow on.
30 August - OKM reports that the naval forces cannot be ready for an operation starting 15 September and that the earliest possibility is 20 September.
3 September - Hitler declares 21 September as the beginning of the operation. It is noted later that weather conditions between 12 and 21 September keep the S-Boote out of the British sea lanes; it is unlikely that operations by the landing fleet could have been undertaken in such conditions.
17 September - Hitler announces that preparations were to continue, but that the operation was delayed until further notice.
26 September - Raeder clarifies that since it is impossible to keep the landing fleet concentrated in the exposed Channel ports, they will be held there until the middle of October and will then be dispersed. Hitler makes no comment or complaint.
If you check Jürgen Rohwer in "Chronik des Seekriegs", which is based upon the OKM KTB and other primary sources, you will find that by 17 September the following had been collected at the ports: 155 transports (700,000 BRT) (another 13 were on the way) 1,277 barges and lighters (another 698 were being prepared or were on the way) 471 tugs (another 49 were on the way) 1,161 motor vessels (another 439 were on the way)
Note first that the number actually available for Hitler's proposed M-Day was 3,064. Of course, that's counting all motor vessels, which included the Vorpostenboot - Patrol boats usually converted from fishing trawlers and armed typically with one or two 8.8cm, up to 9 2cm, and a number of MG, Räumboot - Mineclearing vessels converted from small fishing cutters and armed typically with three or four 2cm guns, Minensuchboot - Either purpose designed minesweepers of the 1935-class (M1-39), which were armed with one 10.5cm and 2 2cm guns, 4 depth charges, and could carry 30mines in a mine laying role or fishing trawler conversions, typically with one 8.8cm and one or two 2cm guns, as well as the Prähme (motorized barges), and straight motorboats - including 200 motorboats and 100 motorized sailing vessels with Transportflotte E's Schleppverbandverband 5, which made up the majority of the motorized vessels intended to move VIII. Armee-Korps of 9. Armee.
Effects of British Air Campaign on the Invasion Ports in September
14/15 September - 7 transports (13,177 BRT) and 2 freighters badly damaged
17 September - 1 freighter sunk, 1 minelayer and T11 damaged
19 September - T3 badly damaged
21 September - 9 steamers, 51 barges and 1 tug destroyed
Total Losses in September: Lost: 2 T-Boot, 12 transports, 51 barges, 4 tugs. Damaged: 9 transports, 163 barges, 1 tug. The numbers put out of action represented 14 percent of the transport fleet, 17 percent of the barge fleet, and 1 percent of the tugs.
Coastal Command was doing anti invasion duties in July, Bomber Command was not bombing the invasion fleet until September, its effort that month was. 216 day bomber sorties despatched. The 30 to Germany (on the 1st to the 8th) were 16 aircraft industry, 10 oil, 3 canals and 1 anti ship. The 186 sorties to occupied countries have 162 anti invasion, that is sorties against warships, merchant ships and ports. These comprised 32 to Belgium, 50 "Europe", 44 France and 36 Holland. Another 24 sorties were sent to airfields (4 Belgium, 10 France, 10 Holland). Sorties against airfields essentially ceased on 8 September, anti invasion sorties began on 9 September.
September, by night, of the 3,088 sorties despatched 1,221 were to Germany, 1,170 to France and 597 to Belgium, 73% of sorties attacked the primary targets while another 14% attacked secondary targets. Remarkably sorties attacking shipping or ports in occupied Europe only started on 2 September and that was 12 sorties to U-boat docks at Lorient, on the 5th 13 sorties were sent to E-boat docks at Boulogne, plus 3 sorties to attack barges at Delfzijl in Holland. It was not until 7 September sorties to Belgium, France and Holland became almost exclusively against ports and shipping, with a further 1,618 sorties despatched to the end of the month. Over 100 sorties per night were sent on the 14th, 15th, 16th, 17th and 19th. It means that of the 1,850 sorties sent to Belgium, France and Holland in September 1,682 were against invasion targets and 57 against airfields. Some 21 of the aircraft despatched to invasion targets were listed as missing, or around 1.25% compared with an overall night bomber loss rate of 1.68% for the month. The ports were certainly defended.