I like it, I wonder if Japan joining the Allies in 1939 would be at the cost of American support, perhaps reinforcing their neutrality, i.e. no lend lease and spooking the Soviets and Germans into tightening their early alliance.
Japans alliance with the UK was ended as part of the Washington treaty in 1921 and I believe it was the end of this alliance and Japans poor treatment by the US in the treaty negotiations that led to much of their, some would say justified, insecurity and resulting expansionist policies. Washington also did over the UK as while they had parity with the US on paper, most of their fleet consisted of earlier construction that while not old enough to be scrapped under the conditions of the treaty were smaller, less capable, pre war designs where much of the US fleet was larger newer and more capable ships. In addition the Dominion navies were also counted in the UKs totals also reducing the core strength of the RN.
No a thought I had in relation to the UK finding a work around on the Washington treaty was that they excised the dominion navies from the RN having each stand as an independent entity. As part of this excision each of the new independent Navies would be gifted older / superseded RN tonnage, say the Renowns to the RAN, Tiger to the RNZN and some of the Queen Elizabeths gifted to Canada. Possibly the large light cruisers to Australia and Canada for conversion to carriers. The RN sheds their oldest, weakest ships and is able to build new treaty ships that are more advanced and capable than the USNs newest and with this new confidant position are able to back Japans quest for parity. This support removes one of the main reasons for going to war in 1941 and reaffirms the existing friendship leading to Japan re-joining in formal alliance with the start of WWII.
This would give you a larger more capable Commonwealth Fleet, a larger more powerful and allied Japanese fleet, a more isolationist US and a closer Germany and USSR, with Japan on the UKs side Hitler would likely have accepted Spain into the fold.