There’s all or nothing and then there’s this…
I personally think it’s a really tough gambit to go Sealion on G1. As has been pointed out above, there is a possibility for the Allies to take back England before G2, thus making the only real results the capture of all British money and the lack of such funds for UK1. BUT, Germany will not have the ability to invade again on G2 so the US can hold the island with 1 INF and be secure until UK2 when they’ll have some money back.
It IS a terrible setback for the Allies; it forestalls any UK defense of India, allows Germany to send a significantly larger army against Russia much earlier than expected (the extra 30 IPCs) and there is no help with the British fighters for the USSR either against Germany or against Japan.
However, it is not a guarantee of victory… the Allies are still strong enough economically to hold and if the US can get things rolling early they can stem the tide. If, however, Germany loses (which is as has been pointed out about 90% of the time give or take) it is der kaput for ze Germans. There is really no way that Germany can recover from the loss of the aircraft that it so desperately needs in order to hold out defensively long enough for Japan to get rolling in the East.
All in all, Germany should stay away from Sealion for the same reasons in game as they did in RL: the inability of the German navy to get significant enough amounts of troops to make invading and holding worthwhile. Rather, concentrate on Russia with hit-and-run tactics using the armour in order to keep softening them up while Japan whittles away in the east until you can go after Karelia.
Don’t throw away Axis air power!!! :D