You have 6 months to get a French registration, more if you go back to the UK regularly.
I have a set of LHD 300 series lights going spare here in Basse Normandy as well as most of a 300tdi manual RHD Disco to go with them.
Make sure you have a bill of sale before you go to get the various forms for getting a French reg no. Don't worry if you don't have a bill of sale, just make one that agrees with the previous owner record and date on your V5C, not that anyone will cross reference it.
Our local Hotel des impots (tax office) won't issue any documents for vehicle imports unless you are registered for French tax, but that's their interpretation of the rules, the vehicle registration people have other ideas about the same rule.
French Controle Technique is a joke. About 5% as strenuous as an MOT test. You'll need a certificate of conformity. Try LR in Solihull for that but I got mine from
EuroCoc.eu - Certificate of Conformity as they are quick.