As it says on the tin, your challenge is to have France own Dover on the British Isles, but nothing else there.
Bonus points if :
-It becomes French during the HYW
-It remains French after the HYW
-It is reconquered by the English during religious turmoils akin to the Reformation.
or
-France uses it as a base to stage an invasion of the British Isles during the age of nationalism.