Easiest way is to extend your French insurance long enough for your initial UK stay before returnuing to France to sell it (LHD cars get very low price here). Don't tell your insurers you are permanently leaving France but just going to UK for a few months. Retain your French address long enough for this purpose, or re-register your car with the address of a friend in France (can be involved if it's in a different préfecture). Every car insurance policy bought in EU is valid throughout EU, but often third party cover only.