To get the maximum defense in the London on Round 2
1. The Russians should have landed their fighters in Moscow on their first turn. They can both land in London on Russia 2. The sub on Russia 2 can go block a med navy by going to SZ12
2. The UK can build 5 inf 3 arm (30 IPCs), which provides maximum ground fodder under the restriction of mobilization and cash, and also land the tank from W. Canada to UK
3. The US can land 2 inf 1 art 1 arm in London through SZ8 on their first go, plus the fighter/bomber from E. US
There is absolutely nothing the Germans can do to stop this massive build-up, except possibly bombing the UK for 6 IPCs.
In the best case scenario for the Germans, they build 5 transports on G1, send their Med fleet westwards, bomb UK for 6 IPCs, and have all aircraft in range of London. But the Med fleet cannot attack London because of the Russian sub in SZ12, so the maximum force they can bring against London is 6 inf 6 tanks 6 fighter 1 bomber (5 built transports + 1 original in Baltic).
The defending force is a composite of the following: 2 inf 1 art 1 arm 2 fig 1 bomb 1 aa originally in London, + 8 inf from build assuming only 24 to spend from a perfect bombing, +1 tank from W. Canada, + 2 fighters from Russia, + 1 fighter 1 bomber 2 inf 1 art 1 arm from US, for a total of 12 inf 2 art 3 tank 5 fighter 2 bomber 1 aa.
In that perfect scenario for the Germans with a perfect bombing, they have right about 1% chance of winning.
Alternately a naval build by the UK like 1 car + 1 dest can deter a German landing, but the UK navy will be destroyed and you’d better hope some transports didn’t get through because the UK doesn’t have the bulk of its land forces ready.