Finding PoDs for a British victory in Malaya isn't that hard, but one that doesn't work is saving Force Z. Even if Force Z can intecept the Japanese landings on the East and North coast of Malaya, it will not significantly influence the mai bn campaign, as the north and east coast then were practically cut off from the important west coast, where all the infrastructure and riches were.
The British plan (Matador) for that reason also didn't put reliance on naval forces operating the South Chinese Sea, but depended on British forces taking up defensive positions on the narrow Isthmus of Kra before the Japanese could enter Malaya that way from Thailand.
That would mean the British violating Siam neutrality, but the British commander of the Far East Brooke-Popham, had in early december been given authority to initiate Matador on signs of Japanese aggression. In the days before the attack he did receive sightings of Japanese convoys converging on Malaya, but he was so scared by Churchill's warnings about not prematurely starting a war with Japan, that he did not dare do anything.
Had he initiated Matador, the British would have been in very good defensive positions that would be very difficult to outflank. The problems in OTL for the Commonwealth troops usually came when they were outflanked. An initial defeat of the Japanese would be a tremendous morale booster for the Commonwealth troops, and even poorly trained troops can be quite effective in defense - as lonmg as they are not outfklanked and things go well.
Matador did prescribe a larger number and better troops than were available in Malaya, so victory is in no way sure, but even so a defensive position on Kra would be very difficult for the Japanese to break.
The defense of Singapore itself, if the Japanese get that far, indeed could be performed a lot better - like preparing it at all. The difficult thing is that the supply of fresh water depended on Johore just across the strait, but with all the troops available, some field fortifications and HE ammo for the artillery it wouldn't be impossible. If not for other reasons then because the Japanese were at the end of their logistic string and had no heavy artillery when the British surrendered in OTL. Arriving just a few days later at Singapore might be enough for the Japanese to fail, as that would allow the regular 18th British Division to be deployed - it had just arrived when Percival surrendered.
The Imperial General Staff had proposed several times in 1941 that reinforcements should be sent. A regular Division, a tank regiment, some AA and about 200 modern fighters would bring the force up to Matador level, and units had been pointed out, but Churchill repeatedly refused to send any more to Malaya - apart from Force Z - which wasn't needed at all, didn't help and was lost.
A PoD where reinforcements are sent might mean offensive ambitions in the Med. and fightersweeps over France being slightly downgraded, but as these operations in OTL had no success anyway, it will hardly effect the allied cause negatively.
A Japanse defeat in Malaya would OTOH have huge effects. Without Malaya, no Singapore, and without Singapore any Japanese conquests in Dutch East India can't be utilised, and the entire raison d'etre behind the Japanse entry into the war is down the drain. If the Japanese go for full scale war they will in a few months run out of oil and the British Empire will combine a liberation of French Indo China and Hong Kong (and intensive support for the Chinese nationalists) with a trade war cutting off Japan from imports. By 1943 Japan will be starving and not capable of waging war.
No communist take over in China and Vietnam, the British Empire last for a couple of decades longer, the Commonwealth is a vital political and economical entity channeling investment and goods between the first and third world, Africa has much better infrastructure when independence comes, EU never happens as the French-German coal and steel union slowly dies away with coal and steel...
Regards
Steffen Redbeard