The re-registration of cars in France is quite straight forward.
There are no import duties payable and if the car is less than 4 years old, if a UK registration V5, Certificate of Conformity, proof of residence and valid insurance certificate is provided to your Prefecture, the procedure can be completed the same day.
For vehicles over 4 years old the car will need a French CT, which is the equivalent of a MOT but it lasts for 2 years.