If you purchased a 15+ year old car via someone like Prestige Motorsport I cannot see them turning round after purchase and saying sorry the rules have changed, bad luck.
If there was an possibility of such a change no doubt Geoff and prestige would clearly advise of it. And even in worst case scenario, if you did buy a car and the ruling did change, the importer would have to refund you the full amount as the conditions of sale had not been met.