Eu and French Law, through the Code de la Route, make the carrying of certain items a compulsory requirement.
In addition, there are items which although not legally required are recommended. Consider the following before you set off. Compulsory
UK Driving Licence
Must be at least 18 with full UK driving licence for the class of vehicle.
If a paper licence, carry passport in the vehicle for photo proof of id.
Vehicle Car registration document V5
Owner's written permission is also required if the driver is not the vehicle owner.
Green Card
Must be able to prove the minimum insurance cover requirement for Europe. Check policy and if required notify insurance company of
your intended visit.
Headlamp Adaptors
Beams adjusted to avoid dazzling oncoming drivers(day or night).
New: UK Sticker or UK with Union Flag identifier number plate.
Sticker should be fixed on the rear of the vehicle or the number plate.
GB and Euro Flag identifiers must be covered or removed.
Warning Triangle.
Must be placed at least 30m behind the vehicle, where safe to do so. Two are required under Spanish law.
Reflective Jacket
Must be Eu approved and kept inside the vehicle(not boot). See news
Spare set of bulbs
Must be able to replace a blown bulb otherwise your vehicle could be immobilised under the law.
Single Use Breathalyser kits(New for 2012)
Must bear the French NF certification mark. See news
Speed Detectors(incl Sat Navs and GPS with safety camera alerts are banned in France and usage carries heavy penalties. See news
Crit'Air Vehicle Air Quality Cerificate(Compulsory) See news

Recommended
Spare set of fuses

The European Good Samaritan Law requires every motorist to provide assistance(where safe to do so) in the event of an accident or incident.
First Aid Kit
Fire Extinguisher