My feeling is that if Fighter Command was comprehensively written down (say to around 200 fighter aircraft) and Bomber Command suitably diminished (cut by around 50%) and the Germans did decide to launch Sealion, the most likely result is the Germans losing anywhere from 10-25% of their initial force of 160,000 men in the crossing and landing, another 15-25% in the actual combat phase while on land and then another 10-20% as they beat an ignominious retreat.
SEA: There is simply no way to prevent the RN from sallying forth, at night or by day, to oppose the crossing. Submarines, mines, the surface fleet and the Luftwaffe were simply not enough to stop the RN braving its MTB/MTGs, sloops, destroyers and light capital elements (6" cruisers) in the channel. All through the BoB period, RN destroyers were running sweeps along the French and Belgian coasts and attacking targets of opportunity, both in daylight and at night.
If a crossing does get underway, as of 14 September 1940 there were the five battleships and aircraft carrier, 16 cruisers and 48 destroyers or corvettes, plus another 700 light armed patrol vessels in home waters. 12 destroyers, three battleships and two cruisers were ready to sail from Rosyth to directly oppose the flotilla.
To fight them, the Kriegsmarine has two old battleships, two modern battleships, five cruisers (including a gunnery training ship) and no more than ten destroyers. The submarine force is mostly going to be deployed in the North Sea or the Atlantic, not in the Channel, mostly due to its shallowness.
AIR: Even a written down RAF is still going to bomb the invasion beaches, ports and the follow on assembly areas in both Southern England and France/Belgium. Fighter Command will take a heavy toll on fighter bomber attacks by Ju-87s and Bf-110s, although F/B 109-E4s and E7s will probably have an easier time of it.
LAND: Sealion envisions a nine day build-up by infantry forces to grant the armoured forces of the second wave enough manoeuvring room for a push on London. This is lunacy of the highest order.
The British defence plan consisted of using fortifications, the Home Guard and static regular army units as a coastal crust to provide initial resistance. Behind this came the semi-mobile formations, again supported by fortifications (over 14,000 by September). Their main objective was to trade space for time and to channel German advances into corridors, allowing the reformed British divisions to chose the ground they fight on.
The nine German infantry divisions that land are going to be outnumbered, outgunned in terms of artillery and armoured support and lacking some of their organic transport support. The bulk of the British Army's armour and artillery was to have been brought in to phase lines inland of the invasion sites, mustered and then used to attack in strength.
What I see is a force that is subject to attack from both the land and the sea by day, by land, air and sea by night. It is outnumbered and under supplied, without significant armour support (the ~250 Tauchpanzers amount to roughly one division of armour) attempting to achieve a wide-front advance against an opponent that is going to meet it with a delaying tactics and resistance concentrated around strongpoints. A few days in, it is going to start to run shorts of bombs, bullets and bandages, while the opposition continues to strengthen. The BA is going to be raining shells on it from the land, the RN is going to be raining shells on it from the sea and Bomber Command is going to be appearing every night and hitting pretty much whatever they can find.
General Franz Halder likened the whole exercise to sticking his hand into a meat grinder. I think he may have been understating the case.
