There's a small group of people lobbying for this sort of category rubbish to be brought into the UK, which would mean modified cars would loose their tax free status, and probably even their black numberplates.
Currently we have it great, any car before 1974 (rolls forward each year), is tax free. Modify the hell out of whatever you want, as long as it's MOT legal. I'm really against anything which leaves us in a similar situation to Germany, it's not something anyone should wish for when we have the current system that benefits so many.