As far as my experience goes, if Germany can get 9ARM into London, USA trying to liberate it gets tricky. For both sides.
Having (and leaving) 9ARM in London can be pulled off IF the luftwaffe still exists to threaten Russia. This is the sort of German victory over London I am talking about when saying ‘sucessfull’. If the luftwaffe is (mostly) gone, Germany is done for by Russian pressure alone.
However, 9ARM is not going to stop USA from liberating London. That can be achieved by Japan’s actions PLUS leaving the Luftwaffe in london also.
Remember, the USA has elements of its pacific fleet to rush into London so if Japan goes down and heavy on India, the US can bring into London US4:
7TRS, 1BB, 3CA, 3CV, 5FTR, 1TAC, 3STR, 7INF, 2ART, 1ARM, 4MECH.
For a total expense of 91 IPCs. USA has 104 IPCs to spend in the 2 first turns so it can even bring more. Germany can ONLY defend against this with all its Luftwaffe in London + the Italian airforce as well. Which is interesting, because then Stalin should smile at the Germans trying to defend against him…
So the first choice rests with Japan; will it play standard OR will it focus on the USA to prevent London from being rescued? The latter gives very interesting allied options as well, as China, India and ANZAC will get rich and the USA can even forego liberating London and help focusing down Japan whilst Russia plays a ‘hold out’ game against Germany/Italy, at which it is VERY capable I might add. And don’t forget Russia no longer even requires its Siberian Hunters in Moscow, so those can stay in the pacific… Staring down Japanese ;-).
The second choice rests with the USA. Going after Japan in turn sequence, the USA has an easier job making this decision and it will be either KJF OR Liberating London. Depending on what Japan does, ofc! IF Japan plays the standard ‘India’ game, the USA can choose to bring down a mean hammer to London to force GE to choose to defend against either the hammer or the (Russian) anvil and it cannot do both. The USA must of course from US3 and on, spend all its income in the Pacific again to prepare for the possibility that Japan turns around and comes for Hawai and/or Sydney. Since Japan is bullying India, this will not be a real threat for at least untill J7 so plenty of time for the US.
Leaves me only 1 thing to say: UK should be perfectly able to make GE have only 1 to 9 survivors in total (including its Luftwaffe) or even win the battle outright! Which makes liberating actions by the US a lot easier OR Russian pressure a lot bigger! In any case, Russia can play an effective hold out game while the USA destroys either 1 axis partner.