My preferred scenario is there is a war in Europe as the butterflies get
really out of control otherwise.
Lion class built or not built depending on how great a threat Germany and Italy are for instance. Development of the entire RAF depending on German/Italian threat as in what kind of bombers and how many?
Development of tanks?
Does Japan even seriously consider going to war ( or just escalating things in China) without Germany and Italy pinning down UK and French forces?
My six Lions can beat your 3 Yamato's in 1944
I have tried to outline (very sketchy) earlier. Things go pretty much as historical (maybe the British loose one less carrier to stupidity by May of 1940) and then things go a little bit worse for the Germans. Not a lot, maybe France lasts a few weeks longer? Maybe not. More French ships Join the Free French? French NA joins the Free French?
To me the important part would be kicking Italy out of NA in late winter or spring of 1941. No Greek adventure.
This reduces British losses of all kinds. May or may not open the Med to ship traffic?
It does give the British some combat experience.
It frees up a lot of equipment (and this is relative, the British built somewhere between 5-6000 Hurricanes by the end of 1941, NONE went to the Far East by Dec 7th 1941) to equip Commonwealth forces in Malaya, Burma, Singapore, Hong Kong and Australia.
This is against the backdrop of the escalating embargos/trade sanctions that brought Japan to the attack stage.
Again, this is just my idea of what was plausible. Possible and probable may lie on either side.
edit> I will say that the whole idea takes a suspension of disbelieve that the US won't get involved after they lead the trade sanctions and embargos and were building up Army and Air force assets in the Philippines and the Philippines occupying a roughly 1000 mile by 1000 miles area right in the middle of the Japanese supply lines/operational area. I have no explanation for that and simply accept it as given in order to get British Commonwealth vs Japan scenario off the ground.