As I understand it, once a UK registered car has been in France for 6 months it is legally deemed to have been imported and, as such, must be re-registered. Prior to re-registration, a UK registered car must have current valid road tax. Failure to comply with the laws of the land could render any insurance cover invalid. Brits who come over to France and ignore the laws just get the rest of us a bad name. No-one, of any nationality, wants to run the risk of being hit by an uninsured car. The problem over here is that the policing of such matters seems to be reactive rather than proactive, meaning that most people will get away with it until they have an accident - at which time the gendarmes will throw the book at them.