There was, at the time, something of a doublethink going on. Whilst publicly friendly, the U.K. was privately discussing scenarios up to and including invading the USSR (Operation Unthinkable).

However, again, it was wartime. And not one of these wars where your professional army fights far from home against an asymmetrically weaker foe. People tend have fairly negative views of dissenters in that circumstance. Look at what happened in America after 9/11.

It was more than that among the Left. Yes, everybody on all parts of the political spectrum on the Allied side tolerated "Uncle Joe Stalin" during the war, as Germans fighting Soviet troops were Germans not fighting their troops, but this was just "the enemy of my enemy is my friend" reasoning. But until Khrushchev's leaked 1956 "Secret Speech" in which the crimes of Stalin were denounced, a large part of the Western Left really thought the USSR was a workers' paradise and that complaints about it were just right-wing propaganda spread by exiled Russian aristocrats bitter about losing their power and wealth. Trotsky of course had been telling the world about Stalin (until he got murdered in Mexico by one of Stalin's agents), but most people likewise assumed that he was just bitter for losing out in the competition to be Lenin's successor.

A large part of the reason for that is that the CIA promoted it _hard_ in the 1950s, including directly financing a motion picture.