That certainly is the Worst Case Scenario.

Measures could easily be put in place to prevent such things. Sure, we now have low cost flights, but if such a problem arose they could be curtailed.

Every time immigrants come to another country it is the end of the world. Be it the French, the Dutch, the Jews, the Carribeans or whatever. All were somehow going to destroy the UK.

Barbaray pirates? A few small ships is not going to bring Europe to its knees. The Yacht sailing minority I am less concerned about. Perhaps they could pleasure cruise elsewhere?

Worst case scenario for those that think Africa is about to destroy the enirety of Europe:

Flights are reduced, ticket prices are increased and people are checked thoroughly, and deported very quickly (next plane).
Police the north coast of Africa to a depth of a mile or so. No more boats!

Come on! We're Anglo-Saxons! Historially from Saxony or supposed to be. Britain has always been a "mongrel race". Sure, we might the country might be altered by immigrants, but isn't that always the way the world changes?