So I've been doing some intense research on the subject and come to the conclusion with help of VOSA that both are perfectly legal.
BUT
You need light board etc and the numberplate of the towing vehicle on the back.
The towed vehicle becomes a trailer so therefore doesn't need tax, MOT or insurance to be on the road, no matter if 4 or 2 wheels are on the ground.
BUT the big thing is
All wheels on the ground MUST be braked if above 750KGs. This actually means 4 wheeled trailers that only have two wheels braked if used with over 750kgs of weight are technically illegal.
It gets pretty hard after Googling and different things coming up here there and everywhere.
Both an aframe and a towing dolly means the car becomes a trailer, not a car as such.
So it's pretty impossible basically to use either unless you have a way of operating the vehicles brakes. This could technically be a solenoid on the brake pedal wired up to the brake light circuit on the towing vehicle but it's all very messy!
So really in all realism, you can't do it unless it's an emergency and you're going to the nearest safe point.
I'm currently in the market for a hefty 4 wheel braked trailer...