Personally, I think Sealion is a waste of time. You will not keep it past turn 4. The US has too many resources for you to keep it. Plus, if you put that much into Sealion, USSR will definitely come right into your back door.
In short, Sealion works but not a smart move to do.
UK makes so little money and no real bonuses that they are almost useless on the European board. Most of the UK moves are in Africa until the US arrives. By bombers if your opponent is planning Sealion. Fly them away to Canada. Once the US takes London back fly them back and bomb the hell out of Germany as the US lands troops.