Warning triangle: if your vehicle breaks down or is involved in an accident, you must give warning to approaching traffic by placing a warning triangel 50 - 150 metres behind your vehicle. The use of hazard warning lights alone is not enough.
A complete set of light bulbs: all lamps, including lenses and reflectors must be in working order, even during daylight hours. Failure to be in a position to replace an exterior bulb could result in temporary cessation of your journey and/or an on-the-spot fine.
Headlamp beam adaptors: failure to adapt your headlamps at any time, day or night, will render your vehicle unfit for use on the road and could invalidate your motor insurance.
GB
sticker: required by law to indicate country of registration.
First Aid kit and fire extinguisher: The European Good Samaritan Law requires every driver to stop and provide assistance when an accident is encountered, providing it is safe to do so. This may require a first aid kit and fire extinguisher and it is advisable to have these items when driving in France.
Reflective vest: Anyone providing roadside assistance or leaving a stranded vehicle should always wear a reflective vest for their own safety. This is now a legal requirement in a number of European countries. (EU ref: RD1428/2003 and Art.162 Cod4 Bis4 Ter - CDS 2003.)
This is a must in France, I work for DFDS seaways in Dover so see many people coming back with a lighter pocket because they have been heavily fined, believe me the police over there do not muck around!
Always carry both parts of your licence and take all your documents to be safe, like mot, insurance etc.
A good breakdown cover is a must, also be aware some breakdown companies will say if it costs more to recover your vehicle than the vehicle is worth they will leave you there and you will pay a lot of money to get home.
Remember there are toll roads so have money available.
Also keep a small amount of cash stashes in your wallet, so only some is in sight to the officer stopping you, they take bribes believe me, otherwise you could see your car gone or be locked up for speeding or making a mistake with the road laws they have, money talks over there with the police. Don't have 100's on show, stash some away and leave enough in there to keep the officer happy and you not too badly out of pocket.
Don't let this put you off, it's very rare, if you stick to speed limits within reason and don't drive like a nob then you will encounter no problems. Also make sure your insurance cover you for everything they would in the UK, go fully comp. insurance will have to lay recovery etc in event of an accident.
