Alright, but what would it take for the UK and Japan to be allied against the United States in ww2? In addition, what would prevent a European front from breaking out?
Alien Space Bats.
The British had to chose between the U.S., a country that was already the richest on Earth, had unlimited resources, and shared both an Ocean and culture with the UK, a long land border with a critical Dominion, and was perfectly placed to take Bermuda, the Bahamas, and Jamaica away from the UK on any afternoon it chose, a country that, incidentally, held UK markers for better than FOUR BILLION 1919 dollars, and Japan, a relatively poor country on the far side of the Planet that was becoming a pain in the neck to British concessions in China, was both feared and disliked by the Australians and New Zealanders.
That is not a particularly difficult decision on any number of levels.
An European war was assured the minute the Great War ended. The "peace" left a series of disastrous border situations, bad blood galore, and multiple enemies just itching to get a shot at each other again.