The U.K. never fought Germany as a lone entity so the question is moot. The Commonwealth is what stood together from the start and many, many lives were spent in defense of the mother country. The U.S.A. was the prodigal older son joining to help its parent and siblings. Russia was an ally only when in its own interest, but a major force as well.
If the USA stayed home the war with Germany would have been prolonged. If Hitler had more time to develop the A-bomb that would have been a total disaster because he had already developed the V-2 missle as a delivery vehicle. If he was able to deliver atomic destruction to London and Moscow the world may have been in his grasp.