From the RIFT facebook page
Without going into issues about driving RHD, vehicles are still generally cheaper in UK than France, so people are still going to be tempted to import them.
However, since 1 January 2021 registering a UK vehicle in France has changed. Apparently you should declare permanently imported vehicles on arrival at the border otherwise they will be illegal on French roads. So, at the douanes, if you already have residency here, you present your CDS showing you’re not a tourist, thus ensuring your passport won’t be stamped, but you’re driving a car on UK plates.
On declaring a vehicle, I believe a customs charge of 10% will be applied to its value, plus 20% VAT. The value is based on the bill of sale when the car was bought. If the owner cannot provide official proof of purchase then the market value of the vehicle will be estimated...however the value in France of a secondhand vehicle could be greater than its genuine UK value and implausible valuations might be challenged.
What about vehicles still on British plates imported to France prior to 1 January 2021? Does the 'Quitus Fiscale' process still apply to them? However permanent residents still owning a vehicle on UK plates will no longer be legally acceptable. You have been warned.