In France, an imported Vehicle needs to be as it left the factory according to the chassisnumber - not your V5.
No fuel type changes, RHD => LHD conversions are an issue to name a few..
The CoC is to be issued by LR France (https://www.landrover.fr/Images/Formulaire-Clients-Land-Rover-imports-VO_tcm286-681213.pdf) for most vehicles - even Series. For really ancient Series you may end up with the Historic Vehicles Ass. and in that case the vehicle needs to be 100 % as it left the factory according to the VIN.
I will not to mention that rather inconvenient minor event to take place it seems later this year that could make it even more fun...
Converting a Series to LHD is not difficult - it just costs money (and time) for some rather obscure parts to do it properly. And some are hard to get like correct wings, handbrake traverse rods, instrument panels, correct Bulkheads, steeringhouse to name a few. Also, not all chassis have a provision for a steering relay on the LH side.
So, if you already have a project vehicle, see what the VIN tells you and decide if that is what the vehicle will look like in the end when it is in France. If it is different you could get into trouble and reconsider. You may want to consider getting a French registered Series and rebuild that - within the French limits.
Before you ask - I live in France, have imported too many vehicles, make a living out of paperwork and it is getting more and more difficult, not to mention expensive, to immatriculate one. Even modern ones.
Bon Courage !