Glancing at the battle calculator, if Germany can bring 10 transports (10 inf, 5 art, 5 tanks), 2 each fighters and tactical, 4 bombers… not unreasonable and hopefully they’d have 1 or 2 more fighters/tacticals, but.
UK can easily add 6 inf, 1 fighter, 5 U.S. bombers, 1 U.S. fighter, and 1 or 2 loads from Canada, and maybe a few inf on UK 2 if they can afford it. (yes, this assumes a J1)
Looks like a total of 14 inf, 1 mech, 1 tank, 4 fighters and 4 bombers is where it finally drops to 50/50.
That means if the UK loses both its Scotland fighter and sends one down for Taranto, and Germany sinks the transport in Canada, then its 100% up to the U.S. (if they’re in the war) to get the fighter and 5 bombers to London, otherwise Germany can take London 99% of the time. Even then, Germany could probably do it anyway pretty easily if they ignored the UK fleet in the med and had close to all 8 of their fighters/tacticals.