Malaya is hugely vulnerable if Thailand and Indochina are held by Japan, in the absence of that Malaya is actually quite hard to attack. Indeed if there is a significant build up towards hostilities Japan faces the danger that it is using its elite carrier based planes and pilots against rather more replaceable yet high quality land based aircraft.
I think much depends on the build up, a sudden offensive favours Japan (but carries the risk of the pulling in other nations), a long term build up may leave Japan in a war of attrition that does not suit it.
I was assuming no Japanese presence in FIC since there must be no European war it to be UK VS JPN one on one. This would mean Japan running convoys from China and a British battlefleet in operating from Singapore.
Japanese carrier superiority would be cancelled by the nearest Japanese bases being in China while Britain would have the opportunity to act behind land based air.
Under conditions where Indochina is still neutral and Britain has time to prepare, their positions in Malaya will be infinitely stronger as opposed to the above. There is still the danger that the Japanese could attempt a landing on the Isthmus of Kra to separate the colony from continental Asia and then muscle their way in with surface assets to move against the sea lanes, but land based air would dissuade the IJN from approaching too close until it was sufficiently whittled down. Under the original defensive scheme of 1940 it was estimated that Britain would need some 600 aircraft covering both Singapore and Burma. Given the circumstances of what they face in this scenario this is seriously inadequate, and indeed if the Japanese achieve surprise it is possible that they could be crippled or wiped out in one stroke. In order to maintain a long-term air deterrent in SEA Britain would need more aircraft, more airbases, and a robust radar detection network spread out over a considerable geographic area with which they can parry Japanese thrusts like they did against the Luftwaffe in Europe.