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For another TL (see signature) the subject of Germany acquiring the capacity for amphibious landings have come up, but may have more general interest.
Lets assume that Germany acquires some level of amphibious landing capacity after the annexation of Czechoslovakia. They build the Marinefährprame of OTL. After the war over Poland they really expand this building and this is noticed.
What would Britain do about it?
Concentrate on defending the other side of the channel coast as OTL?
Advocate the move to a more northern defensive line in Benelux when the time arrives?
Build coastal defenses and face the French criticism that they are prepared to abandon Europe?