The allies can always make sealion next to impossible if they really set their mind to it (UK build infantry, no Taranto, bring ships home, get Canadians, prepare US airstrike). If they do everything in their power to make it costly and Germany still goes for it, then Germany will be in bad shape after the sealion “victory”. In that case, losing London is bad but not necessarily a total disaster. Suppose the opponents don’t really like playing Japan and they’re not very good at it. As long as USA, USSR and remnant of UK can hold the line against germany, the axis are placed in the crummy position of having to win as Japan. In that case, it might actually be preferable to intentionally allow a (costly) sealion in order to put the onus on Japan. If its a team game and the guy playing Germany is more skilled than the guy playing Japan, then coaxing them into a costly sealion might work out OK for the allies.