OK , France police are very 'picky' folks so watch for them , they will fine you 1000euros on the spot for even minor offences and give you a court date to show up to (3 months later) to get a refund of part of that fine.
Do not bring any radar detectors , they are utterly illegal here (passive ones like on tomtoms are ok)
You will need a reflective vest in the car at all times (and if you don't have one for each passenger they must remain in the car if you stop at the roadside for a breakdown etc).
Reflective red triangle is mandatory as is a bulbpack. The bulbpack itself isn't written in law but a defective bulb must be replaced instantly so carry a bulb pack or risk the 1000euro fine.
Oh yeah , don't worry about having enough money to pay the 'on the spot' fines, they will escort you to a cashpoint to withdraw the cash and will impound you car and probably arrest you if you can't pay.
Do not , 100% do not , 'pretend' you haven't seen that a french police officer meant you were to pull over , 'sorry officer i thought you meant the car in front'. Not that you would as they are armed and look way more menacing than our police but, they will go mental for that and you will at a minimum lose a full day of your holiday for 'insulting' them like that (probably at the roadside having every last piece of paperwork and the car inspected to teach you a lesson).
You have been warned. French police are so p'd off with Brits flouncing their laws and then the DVLA not assisting them later that they will be quite harsh.