Given the situation in the Atlantic and MTO by end of 1940, is there anything Britain could have reasonably have done throughout 1941 to better prepare for the coming war with Japan?
The British Govt & Chiefs of Staff get a lot of criticism but they had very limited wriggle room with Britain fighting alone at the end of 1940 and fast running out of money. Resources had to be prioritised to those theatres where fighting was actually going on. Firstly a quick look at what Britain faced around the world from early 1941.
The Battle of the Atlantic was just a constant drain in this period.
The Luftwaffe Blitz on Britain went on until May 1941. The threat of the Luftwaffe switching to day raids had not gone away. The threat only scaled back significantly when the Luftwaffe moved east for Barbarossa.
The CoS only began to feel comfortable that the threat of invasion
in 1941 had passed around Sept 1941 (the weather would be against the Germans after that), 3 months after Barbarossa.
After Barbarossa, the speed of the German advance generated a real feeling that a Soviet collapse could occur. If that happened a potential invasion would be back on the table for 1942. So keeping the USSR in the war became a priority. So the Russian convoys started in August 1941 to supply the material to keep them in the war (tanks, Hurricane fighters included) but at the same time tying down a chunk of the RN. By the time the various Lend Lease agreements had been negotiated & agreed, the USA was unable to supply virtually anything to the USSR in 1941.
That threat of a Soviet collapse also impacted the Middle East via a potential need to move into southern USSR to deny / destroy the Caucasian oilfields to deny them to Germany. And the threat of Turkey joining on the Axis side also existed. Those needs / threats meant transfer of troops & aircraft from Britain to the Middle East not North Africa.
There is a tendency to lump everything from Gibraltar to the Persian Gulf into the term "Middle East" or "MTO". But it really needs to be broken down into the war in the North African desert & Greece / Crete on the one hand and what was going on elsewhere in Palestine, Syria, Iraq & Persia / Iran on the other. 1 April 1941 saw a coup d'etat in Iraq which required British intervention to protect its interests in the oilfields there. Limited Luftwaffe support for the Iraqis came via Syria, necessitating invasions of Syria / Lebanon in June & July to eliminate that threat. Much of the effort was taken up by forces deployed from India. There was then the brief Anglo-Soviet invasion of Persia / Iran in late Aug to protect the southern supply line to the USSR and the oilfields and most importantly the large refinery at Abadan at the northern end of the Persian Gulf, again something which required resources & tied men down.
In the Western Desert of Egypt the war with Rommel was see sawing back and forth but started unexpectedly early.
Then there was Greece. A political decision to back the only other country fighting the Axis in early 1941 and also to show the USA that Britain was still in the fight. It would also have provided potential bases to bomb the Romanian oilfields. Initially reluctant to seek help, the Greeks finally agreed in March. Then, with Rommel not expected to attack for some time, troops and aircraft were withdrawn from the Desert & elsewhere in the Middle East and sent to Greece in April, just as both Greece & Yugoslavia were invaded. Then retreat via Crete in a disasterous campaign that lasted until the end of May 1941.
The arrival of Luftflotte X on Sicily in Jan 1941 & then later in Greece changed the whole balance of the war in the air and at sea and cost the RN dearly in ships both lost and damaged in the first half of 1941.
Away from the U-boat threat, German raiders were being dispatched to pick off merchant shipping around the world, necessitating an RN presence in the South Atlantic & Indian Ocean to patrol the shipping lanes. Hundreds of thousands of men were being moved around the world in troop convoys that had to be protected (tying down old capital ships and cruisers in the process). S & G had broken out into the Atlantic in Jan-Mar 1941 in Operation Berlin and then holed up in the Biscay ports so tying down RAF & RN resources. Bismarck & Prinz Eugen followed in May with well known results. The threat from the last named three ships didn't disappear until the Channel Dash in Feb 1942 saw them return to Germany.
There was some bright news however. Lend Lease started in March 1941 allowing ships to be repaired in the USA safe from bomb attack, and supplies to continue flowing without immediate payment. The USN agreed to shoulder part of the Atlantic convoy escort burden, even though it wasn't at war with Germany.
It also needs to be borne in mind the length of time it actually took to prepare & move assets from Britain to the Middle & Far East. From Britain everything had to go via the Cape of Good Hope and South Africa. Once a decision was taken to send a division it had to be prepared, overseas leave given and moved to a port of embarkation for loading. Shipping had to be found and allocated by the MOWT, a process that generally took two months or so ahead of the expected departure date. When a convoy sailed it generally took 6-8 weeks to arrive in the Middle East or Far East. The main port of arrival for the Far East was Bombay on the west coast of India. That often had to be followed by a long train trip across the country.
So once a decision is taken to reinforce Malaya it would take 3-4 months before those reinforcements arrive, unless they could be obtained from India or Australia. Once they arrive Major Stewart who trained the 2nd Argyll & Sutherland Highlanders believed it took them 6 months to acclimitise to the jungle (not much jungle where most of the troops in Malaya came from). The Argylls were reckoned to be one of the best trained jungle units in Malaya in Dec 1941, but they had been there for 2 years and had been trained hard in the jungle.
And much changed during 1941 to increase the threat level. While Japan had occupied Northern FIC in Sept 1940, it wasn't until the end of July 1941 that the occupied the southern half and so bringing their air forces within bombing range of Singapore. This seems to have been largely ignored despite there being intelligence available on Japanese aircraft performance. But given the the timescale for a reinforcement noted above, it is unlikely that, even had a decision been taken to reinforce and the units found, those reinforcements would have arrived in time.
So lets look at what was sent in 1941.
The main infantry units were the 9th (2 brigades only) & 11th Indian Divisions and the 8th Australian Div along with the Singapore Fortress Division & a reserve in the !2th Infantry Brigade. The latter two units had been in theatre since 1939 AFAICT. While the two Indian divisions had been formed in Sept 1940, some of their constituent Brigades were older and some younger. Most seem to have arrived in Malaya in Spring 1941 but one Brigade didn't arrive until Sept. The 8th Aus had formed in July 1940 with the intention that it should go to North Africa. Again it was subject to reorganisation in 1940 before it sent 1 brigade to Malaya in Feb with the second arriving in Aug (the third remained in Australia).
Undoubtedly more could have been done to train at least the earlier arrivals amongst these troops in jungle warfare.
As for the RAF, they had ordered 170 Brewster Buffalo Mk.I in early 1940 specifically for this theatre. But they only began to arrive at Singapore in Spring 1941. 5 new squadrons were equipped to operate them between March 1941 & Oct 1941 (67, 243, 21 RAAF, 43 RAAF & 488 RNZAF). Each was built around a small nucleus of experienced personnel in squadron & flight leaders, with the rest direct from OTUs in Britain, Australia & New Zealand. That was no different to what was happening with the rest of the squadrons being formed at home in the period as the RAF rapidly expanded. Beauforts were on order for the two Vildebeest TB squadrons from the Australian production line but production there had been delayed. It had been planned to send new Baltimores to the Far East but the decision was taken to route them to the Middle East instead. The RAAF had already supplied 2 Hudson squadrons in 1940 but had nothing else to offer (3 squadron in the process of converting to type in Dec 1941 & 3 others spread from Timor to Rabaul).
It was only from Spring 1941 that various US aircraft types began to enter service with squadrons in the Middle East. Tomahawks, Marylands & Bostons. Kittyhawks & Baltimores were only arriving at the end of the year. The RAF there was still terribly reliant on Hurricanes & Blenheims.
As for the RN, it was doing its best to create a fleet to deter the Japanese. With the US shouldering more of he burden in the Atlantic, the old R class battleships were freed up to refit and be sent out, working their passage en route as WS convoy escorts. Repulse had made the same journey starting at the end of Aug. A plan had been formulated to add Nelson, Rodney, Warspite & Ark Royal around April 1942 and have the Med Fleet at Alexandria based around QE, Valiant, Illustrious & Formidable in the same time period & able to transit the Suez Canal if required in the IO. Sending PoW and having it join Repulse to form Force Z was a decision only taken in late Oct.
But so much went wrong with that plan. Barham & Ark were sunk in Nov. Nelson was torpedoed in Sept. QE & Valiant put out of action in Dec on top of the loss of PoW & Repulse.
Against that background it is very difficult to see what could have been done to help Malaya without a major change of strategy and a different political outlook.
Edit:- The campaign I forgot to mention was that in East Africa against the Italians. It rumbled on from June 1940 until the end of Nov 1941 but surviving Italian troops carried on a guerrilla campaign until Oct 1943 when news of the Italian surrender finally filtered through to them.