taken from aa web site
Towing on an A-frame An A-frame or recovery dolly can only be used legally to recover a vehicle which has broken down. If you tow a car that hasn't broken down using an A-frame or dolly then the law treats the combination as a trailer which must meet the appropriate braking and lighting rules. Trailers below 750kg don't have to be fitted with braking systems, but if a braking system is fitted to a trailer of any weight – as is clearly the case for a car – then the braking system must operate correctly. This is not possible for normal systems fitted to cars, particularly the brake servo, which would not be working unless the engine was running. Trailer regulations also require the fitting and use of a secondary coupling system to ensure that the trailer is stopped automatically if the main coupling separates while the trailer is in motion or, in the case of trailers, up to a maximum mass of 1,500kg that the drawbar is prevented from touching the ground and the trailer has some residual steering. To comply with lighting regulations while being towed, the car (in its capacity as a trailer) would need triangular red reflectors and the number plate of the towing vehicle. The simplest and safest way to tow a car behind a motorhome is to use a car transporter trailer. This can be done completely legally as long as you make sure you don't exceed the towing car's Maximum Permissible Towing Mass and the Gross Train Weight (maximum permitted weight of car and trailer combined) specified by the car manufacturer.