Gibraltar on the Channel
IMHO, Gibralter can be supplied from sea while some-what protected from land-guns, can only be attacked across the narrow, flat 'Neck', and has the eponymous Rock to overlook attackers...
Unless you can re-arrange Calais' geography to provide a natural fortress, you must create a star-plan trace extending many miles-- Like Antwerp ?? By 1940, it might resemble the Belgian Forts or Maginot Line.
FWIW, there's ATMahan's 'Rule Britannia' on new HP&CA site with one possible consequence of a 'British Calais'. Um, I don't know if you must register to read, but linky follows. Beware wrap...
http://www.tboverse.us/HPCAFORUM/phpBB3/viewforum.php?f=15&sid=55e69ac9a60142844bf7c4d9aa7263fd
Wild, wild thought: A Channel Tunnel was begun in 1880 --Go Google !!-- then abandoned over concerns about security. But if Calais was a fortified British holding, that's a different ball-game...