Calais was English for two centuries from 1347 (until the French stole it back in 1558) and only last week the Pas de Calais region - with a view to making a few bucks out of the 2012 Olympics in London - was reasserting itself as part of southern England. So Calais taking over the Dover Harbour Board is no problem!
If it was Boulogne or Dunkerque trying to take over . . . then there WOULD be just cause for British outrage!