>>18259296The USSR was not an enemy of Nazi Germany at the outset of the war. Britain entered the war against Nazi Germany while there was a pact between Hitler and Stalin, remember. So they were siding against both, essentially, until Hitler betrayed the pact turning the USSR into an ally of convenience. By that point, the war against the Nazis had become so desperate for Britain that they were willing even to extend help to the communists if it gave them a shot at beating Hitler. They didn't have the luxury of being overly choosy about allies at the time.
Also it's not as though Britain planned on being materially exhausted by the war and unable to mount any resistance to communist domination. That's just how the war shook out for them. It left them critically weakened, and the only ally they could turn to was the United States.