Since you do not specify a time period, let me list a few general reasons.
A) Britain is to the North, meaning that they could easily invade the French coast.
B) Britain and France have been at each other's throats for centuries (100 years war, Napoleonic wars, Seven Years War, American Revolutionary war, etc.), and the British have won most of the time.
C) The British have always had a superior navy to that of the French, which can lead to some fear if say and embargo happened.