I just participated in a few global games, both that had a sucessfult sealion G2. I played Japan in one, and watched the other game.
UK was undaunted by a single German transport in my game, and the German airforce plus a bombardment from a battleship and cruiser were enough to take the UK on G2. A single German tank and 1 bomber survived. In the other game, the situation was similar, though the UK player had less defending England at the time.
The results went both ways:
In my game, the German player left the tank defending England, knowing that the U.S. would be able to retake London no matter what he put there. Of course he could slow the U.S. by fortifying, but the IPC gain from the UK, plus the IPC loss from England collecting no money until round 4 he considered enough of a victory to be content losing England, instead spending his money on rebuilding his airforce, and submarines (along with some Italian naval power) to help control the Atlantic, as well as ground forces to invade Russia.
In the other game, Germany heavily defending London… by the time America was in the war, Germany had nearly 20 infantry in England, as well as other troops.
The reults? Well, in my game, Tthe U.S. completely ignored my Japanese, allowing me to fight the pacific battle unopposed. UK Pac did a fine job trying to liberate China, but it was only a matter of time before I overpowered them. The U.S. was ready and able to retake London, but not before the superior German numbers were standing on Moscow’s door. The game was eventually called in favor of the Axis after the fall of Africa and all but Moscow. The U.S. fleet was hammered by about 8 submarines (all lost) and many aircraft (all survived).
The other game - the U.S. committed heavily to both theaters, challenging the Jap fleet in the Pacfic, and building a sizable landing force in Europe. The Atlantic force was quite patient, allowing Germany to build in the UK. What do they do? Go straight for France and Berlin, ignoring London. The Russians were able to push forward as well, as the Germans spent too much in trying to hold London itself. This game ended with an Allied victory.