Let's go right to the meat. Would GB have defeated the Nazi's if the US military had stayed out of WWII? And don't forget that the Lend-Lease agreement allowed the US to support the Allies (including GB) before we even officially entered the war.
Royal Navy should in itself be a clue to the victory of why we won!
The English Channel
Nobody ever points out when they talk about WW11 about the role that this fifty mile stretch of water played. A proper organised ground offensive by the Germans was never possible unlike what happened in continental Europe and USSR.