For Churchill, it was a matter of necessity. There's a famous oft-repeated quote (the National Churchill Museum lists it, but with no source) that Churchill allegedly said about the Soviet Union's entry into the war: "If Hitler invaded hell, I would make at least a favorable reference to the devil in the House of Commons."

There is an oft quoted axiom that US materiel, British intelligence, and Soviet manpower is what ultimately defeated Nazi Germany.

While lacking in nuance, the truth of it is roughly approximate. Russian military casualties are generally accepted at 8.7 million (G. I. Krivosheev. Soviet Casualties and Combat Losses). By comparison, the UK lost 450,000 servicemen, the US suffered just under 300,000 and the French lost even fewer. So many Westerners just don't realize the scope and brutality of the Eastern Front in WWII. I assume you are familiar with the Lend/Lease aid program that FDR provided to help equip the Red Army.

As far as anticipating post-war intentions, both the UK and the US knew that the USSR would eventually become an enemy (hell, they even drew up plans-- Operation Unthinkable-- to invade the USSR directly after the fall of Nazi Germany. General George Patton was a proponent of the idea), but at the time neither country was in a position to turn down the Red Army's help in defeating the Nazi war machine.

However, most of Stalin's atrocities (purges, gulags) were not entirely known in the West until well after the war. They were godless commies, etc, at the time, sure, but it was not generally known that Stalin was systematically imprisoning and executing so many of his own citizens. Not that it would have made a whit of difference if said atrocities were broadcast loud and clear from London to New York. We would have worked with him anyway.

Finally, ironically enough, it was Hitler himself who pushed the Soviets into the Anglo alliance. If he'd just split Poland with Stalin as per the Ribbentrop/Molotov agreement and stopped there, events could have played out much different. Once Hitler went ahead with Operation Barbarosa, he pretty well pushed Stalin/Anglos into a "enemy of my enemy is my friend" situation