Britain never regarded the Soviet Union as anything other than a friend and ally once the war started

Really Jim?
A British force was sent to fight the Russian invasion of Finland in 1939, but were too late.
In 1939-1940 the British leadership was sympathetic to Finland in her war against the USSR (the Winter War), yet could not afford to alienate the Soviets while an attack from Germany was imminent. The USSR however supplied fuel oil to the Germans which was used for Hitler's Luftwaffe in the Blitz against the United Kingdom. Because of the Soviet non-aggression pact with Germany, Hitler's troops were able to overrun most of Western Europe in the summer of 1940.