You are correct that if Germany builds a carrier, destroyer and submarine on G1 and brings in some planes to scramble from Western Germany it is very hard for the British navy to drive them from the sea on UK1.
The German assault on England via Sea Lion on G3 is not automatic. They have to successfully complete two strategic bombing raids and repel the British navy on Uk1 with little or no loss to their air force. But you are right, if they do, they only face 16 Infantry (on average), 1 Artillery, 6 Fighters (assuming they lost none in interceptor duty), 1 Tactical Bomber, 1 Bomber, and 4 Anti-Aircraft Guns. The Germans will bring 22 ground troops, 5 Fighters, 5 Tactical Bombers and 2 Strategic Bombers. Even with the Anti-Aircraft Guns the Germans should roll over the British.
But I don’t see any way to stop Sea Lion. Unless the game designer intended for Sea Lion to be a weak strategy because of the removal of the German NO for London and the new rule allowing Russia to invade if London falls.
I just have not played any games where London fell under the new rules so I don’t know how the Russian offensive strategy will play out vs Germany and the Balkans on R3. As soon as I do, I will let you know.