My defense for Sea lion is no defense, when I see signs of Sea lion I begin planning my offense.
I build a battleship and a destroyer UK1 in Canadian waters, and a Aircraft carrier and a cruiser UK2 also in Halifax. I consolidate any and all surviving ships from the G1 blitz and fly all 3 fighters out of England and land 2 of them on my new carrier, just around the time Germany has dropped 10 transports in the Channel.
Meanwhile the US is building a landing force big enough to liberate London not long after it falls. They will need their 1 cruiser and as many transports and land units that can be built on all 3 minor ICs in 2 turns, plus all their land units and 2 transports from the West. A Battleship or Aircraft carrier from the West is helpful but not necessary (best to leave US capital ships in the Pacific to deal with Japan) due to the big boat support from the UK in Canada.
I do all of this to coax Germany into buying sea units that will eventually be destroyed, to allow Germany an easy invasion during turn 3 which will bring the US into the war early, and to save as many British units as possible.
I find that a planned liberation is a lot less risky than a planned fortification, as far as preserving forces are concerned. Imagine using all your starting forces and spending 2 turns of IPCs on your capital just to lose it all. So now you have no Capital, zero money and very few units in Africa to get back into the war.
Now imagine losing your Capital, having zero money, but now in the Atlantic you have a battleship, a full carrier, 2 cruisers, 2 destroyers and a transport ready to merge with an American fleet of 1 cruiser escorting 9 full transports. The Germans will need to get over to the eastern front before the Russians start walking on Poland and they know that if the lose London they will never get it back, not with the US helping out.
So when I see Germany building an Aircraft Carrier, a Submarine and a Destroyer G1. I know exactly what to do.