exactly 50% if UK goes all out on defense. This would mean, that Germany won’t survive with much left (airforce + groundforces). Russa will be a huge threat and the US can relatively easy free the UK again. If the UK plays bad, then sealion should be quite likeley. This gives Italy some time to get going (so Italy is not boring to play) and if Germany doesn’t proceed with sealion turn 2, but goes Barbarossa, then the UK can fight for Afrika (give Italy a good fight) or try to invade Norway and help out the USSR.
As the UK was the sole opponent of Germany and Italy for a quite long period, it shouldn’t be unlikely to defend the UK, even if Germany is going sealion at full throttle.
just my 2 cent.