French regulations are quite clear:
1.2.1 REGULATION CONCERNING CIVIL AIRCRAFT WHICH CARRY OUT INTERNATIONAL FLIGHTS
GENERAL
Any overflight of the French territory with or without stop, should be performed in compliance with the Civil Aviation regulations applicable to this territory.
All aircraft arriving from abroad and entering the French territory should perform the first landing on an airport provided with Customs, Police and Sanitary controls. Also, all aircraft leaving the French territory abroad should have their last stop in the French territory on such type of airport.
https://www.sia.aviation-civile.gouv...09_GEN-1.2.pdf
You can fly in or out of France from/to the UK only through a Customs airfield. Ignore this rule and you risk getting an unfriendly call from the DGAC or CAA.