I know that cmdr Jen talked in depth about taking London on G3 (Before Alpha +3.9)
With the new settup (Alpha +3.9) you may want to wait till G4 (This may be better timed to a J4 Crush Calcutta anyhow)
And then the USA will not enter the war prematurely.
I do know that if Germany does it right there is absolutely nothing in UKs power to stop London from falling (Barring erratic dice of course)
This is fact, set the board up yourself and try it.
Do everything that UK can possible do to beef up London on its turn 1,2 and 3. Germany will still take London on G3 if Germany knows what its doing.
(Which includes heavy SBRs)
And noone can help UK in the Atlantic theater till round 4. USSR cant send planes, USA cant send planes either, till round 4 (Which is after Germanys 4)
And the USA will not be able to take London back till atleast its turn 5. USA moves into position on its turn 4, then next round assaults London. So Germany will have one round atleast to beef up defenses.
If UK doesnt assume that London will be attacked and doesnt rally up fighters and the tac from the med on UK1 and instead uses them in the med or N Africa then they will be out of reach to protect UK on UK2 and Germany can take advantage of this and hit on G3.
So UK must assume that London will be assaulted by G4.