The best first round purchases for Germany always seem to involve a carrier. If Germany builds a carrier and say 4 infantry, indicating he is definately going barbarosa, and Germany sees you build this, he should imediately start calculating how many transports to build (my guess is as few as 7 more to give you 8 total could do the job) and planning to hit London on G3, with nothing going into London on B1 Germany should be able to tear up London on G3 without much problem.
As Britain the first thing I would do is make sure London isnt going to fall, then after G2 and no transport builds by Germany I’ll start worrying about the Med and Italy.
Britain is going to have 3 inf and 3 ftr + 1 fr inf 1 fr fit, + maybe a fighter from Normandy add in 10 infantry built on turn 2 and your talking about 14 infantry and 5 fighters at best defending. If germany hits with 8 tanks 8 infantry and 7 aircraft (I’m assuming some losses to the British fleet and AA), Britain is going to fall.
I think Britain needs 9 infantry and the fighters from malta and gibralter to be safe. Protect London first then go after the Med.