A quote from Daniel Hannan at his Telegraph blog, 27 October 2008

France is casting acquisitive eyes at Wallonia. A majority of French voters now favours absorbing the Francophone province in the event that Belgium breaks apart. […] I feel the British ought to hold out for something in return. We created and guaranteed Belgium largely to stop the French getting their hands on it. Honouring that guarantee was perhaps the single most costly thing we have done in our history. If we are now being asked to reverse our policy, it seems only fair to demand compensation.

How about Dunkirk? The town was English until 1662, when Charles II, that early Europhile, sold it to the French in return for a subsidy that he hoped make him less dependent on Parliament. Returning it to British sovereignty (I'm sure we could offer it some attractive, low tax, free-port status) would efface one of the more shameful betrayals by an English monarch of his country's interests.

Or if the French regard Dunkirk as too high a price to pay, how about scrapping the Common Agricultural Policy?