The success of a G3 sea lion is highly dependent on what the UK does, because a few units in or out of the fight can really tip the odds. Â For example, if the UK builds all infantry both turns and does not do the sz 97 attack then they can have 5aa, 26inf, 1mech, 1tank, 5ftrs and 1bomber defending your attack of 11inf, 2arty,9tanks, 4ftr (assuming no losses and 1ftr covering the fleet) 5tac and 2bombers the odds of winning that fight are less than 20%. Â On the otherhand if 2ftrs and 1bomber are in Med due to the sz97 attack and the 1inf and 1tank could not get over from Canada, then the odds are closer to 75%.
One thing you can do, if you made the big investment in transports and the strategic bombing did not work out as well as you expected, and you find the odds are prohibitive, is to land your 22 units on Scotland on G3 and then attack London on G4 with those plus another 22 units (just remember to buy 10 inf for Berlin on G4 so you are ready for the Russian on slaught).
Another option is to buy 2 transports on G1 instead of the sub and destroyer. Â That way you can land 6 units in Scotland on G2 that the UK either has to attack (drawing units out of London) or are additional units for your attack on G3. Â Just make sure your navy is strong enough not to be sunk by a UK2 attack.
On the subject of US bombers, the real threat they pose is not so much as defenders of London (although as noted above a few units can make a difference) but as destroyers of the German navy and your 77 ipc investment in transports. Â 5 bombers and 2 carriers worth of planes flying out of sz 102 can take out most German fleets (which may mean you need to make a naval buy for Normandy on your attack turn as Variance suggested) or you need to make sure that he cannot land in Erie or Scotland (this is why UK players often send the mech to Erie on UK1)