It's a gray area,
as a trailer it's over the 500kg limit for being unbreaked,
breaked trailers have to by law have a 50/50 break distribution between axles (cars are closer to 70/30 I believe).
To run it as a breaked trailer it needs to have an overrun hitch that connects to a cable operated unit that presses on the break peddle of the car (shill not a 50/50 split).
So they cannot be classed as a trailer.
If you have someone in the car doing the breaking the car must be taxed, MOT'd and insured.
Tomorrow I'll point you in the right direction as to who you should ask.
Julian, I believe RV's come under commercials so have a higher unbreaked towing limit, this is why the cars are always small micro cars.
