Soviet Union without US supplies would have not been able to stop Germany - it would have been starved to death especially once Ukraine and Belarus were lost - just like the famine twenty years before and once again was saved by US food also.
If Japan had attacked from the East, they would have also lost the production site outside Nazi range, while forced to fight and split forces on two fronts very far apart - like US had, BTW, requiring a gigantic logistic effort to supply its troops and allies worldwide. Actually, the importance of logistics in such kind of war is greatly underestimated - you can't fight big battles if your men are starving, your vehicles are too few and without spares, few ammunition, and little fuel.
While after Britain defeat Germany would have had easy access to worldwide supplies - especially the much needed oil. It would also have given Germany access to the British industrial power - while the German plants would not have suffered bombings, as Soviets had no long-range bombers (until they stole a B-29).
And I wouldn't rule out once defeated part of Britons would have not joined the Nazis... especially against the Russian.
Too many possible outcomes....
Biting the hand that feeds IT
