While I agree with the general sentiment that players should have good leeway to play the game they want to, I also appreciate the idea of making things harder for Germany. While we will never know for sure if it would have worked this way, it is a fact that the Germans and the Russians did not trust each other. If Germany were gaining such total accendency as taking London would represent it is not implausible that Stalin would say "Better now than later!" and attack when Germany was unprepared. Paper pacts didn't stop Germany, no reason to think they would have stoped the Soviet Union.
If a Sealion Operation were made by Germany in 1940, an American intervention is more probable tham a Soviet one.
In 1940, Stalin was very happy to see German fighting with Ocidental powers, to intervene. Even if German took London, they will not defeat the British Empire, The Royal Navy and the Commonwealth.
Stalin was very afraid of the weaknessess of Soviet army, after the officers purge, and the fail in the Finland Winter War.
He started to build defenses against a German attack, which he think as probable only after 1942.
Stalin was very happy with the non agression pact, which gave him half Poland, the Baltic states and Bessarabia, without a single shot.
In 1941, he received sucessive alarms of German troops on his bordes and an preparation attack. These alarms were send by British embassy and sovietic intelligence forces.
He did not listen to them, saying that England was not trustable. He used to call "traitors" the ones who said that German will invade soon.
Stalin was very happy with the War, and send germany large supplies of oil, food, coal and metal. This commerce was more and more intense after the fall of France.
When Stalin finally was informed about the invasion, he fell in depression for two weeks in his "dacha".
Stracted from the book "Warlords", from Simon Berthon.
When Germany advances through west, the game could simulate Russian responde through a series of events in which the Soviets build fortresses, fullfill them with reserve troops in the rearguard, and transfer vital industries to the east, which, in fact, they did.