You can get legal a frames i believe. The have a slider in the frame just like a normal trailer and on overrun the pull on the cable which is attached to a device that fits onto the brake pedal of the car being towed and uses to rear cars brakes thus making the vehicle being towed braked therefore satisfying the anything over 750 kg has to be braked rule. Im sure the towed vehicle is classed as a trailer as long as its numberplate is covered and the car that is towing it has its one displayed