Unviable scenario because if Germany had invaded GB, the USSR would have invaded Western Europe through Poland and Romania. The reason that Operation Barbarossa was such a success is because Stalin had his entire strength forward deployed in preparation for invasion as soon as a German assault on GB was underway. Many historians believe that Operation Sea Lion was simply a strategic deception intended to do exactly what it did. I've read that the Soviets actually noted and were alarmed by German preparations along their border and were assured that the build up was simply an attempt to decieve the British. The Germans HAD to invade the USSR - they were existential enemies and both couldn't survive. When people say the Germans made a mistake invading the USSR they fail to take into account that Communism was not a peaceful doctrine. Look at Soviet actions in Spain, Hungary, Finland, the Baltic States, Eastern Poland. Stalin was every bit as brutal and expansionist as Hitler. The German invasion of the USSR was entirely necessary to eliminate the threat, not just to Germany, but to all of Europe.
The actions of the Japanese and their relationship with the USSR is interesting. The two countries had a nonaggression treaty and maintained an active trading relationship until the day after the Bomb was dropped on Hiroshima and the USSR invaded Manchuria and the Kurile Islands. Most Americans don't realize to this day that, while we were supporting the USSR through Lend-Lease, the USSR was trading raw materials like coal, lumber, grain, iron, leather to the Japanese. Of course, by that point FDR had our government so filled with Communist spies that NOTHING bad about the USSR was allowed to come out.