Interesting links on the proposed German invasion of England…
I have quite a few books on Sealion.
The overall conclusion of most historians (and what if experts) is that Sealion could never have succeeded… having said that the whole what if of Sealion makes for interesting games.
In the past I had quite a few games with Home Guard versus German paratroopers, great fun.
I once wrote an alternative Sealion, Otaire de Vigneur, the French invasion of England circa 1940. Though the french strategy of eclair was never taken seriously by British officers in the field.
If you consider the logistics of D-Day, the allies built two ports (Mulberry), the Germans would have had to capture an English port.