I have done something very similar to this. My buddy knew I always go for Moscow first. This game I planned to. I bought 3 transports on G1. Made the med attack with the sub and med fleet.
G2 masses Germany navy in SZ7 took London. My buddy does the Ho-Chi-Men trail through the US to deter Japan from steping foot on the US mainland. In addition to this he built a navy once they got to E Canada (2 rounds later) he took and held London. I never got it back.
Had I not done some fast thinking with Japan on the US W coast Germany would have been toast.
I had a topic started about conducting Sealion every ones 8 ball said no b/c Germany needs help from Japan. So I think you would have more luck with a Canadian Sheild then you would with a Sealion.