We could not have relied on Russia to prevent an invasion of the UK, Stalin formed a pact with Hitler to divide Poland between themselves. Hitler wanted access to the Polish Corridor so he could unite East Prussia with Germany. There after Stalin also took the Baltic Nations and Romania. This is what prompted Hitler to attack Russia.
Had Chamberlain not made a pact to act against Hitler if he invaded Poland we would have been quickly overrun by Germany. We were still in a weak position as we had not built up sufficient arms (unlie Germany) in order to fully resist. Hence the blitzkrieg and the evacuation of our troops from Dunkirk.
The US supported us by providing arms (lend lease)and later by joining in the war against Japan and Germany. If they had not we would be a different country today.
If the UK ,US and our Commonwealth and European Allies had not fought then we would still maybe be under a totalitarian regime.