A lot of it depends on the copper on the day, LEGALLY you shouldn't use them anyway.
An A-frame or dolly can ONLY be used to recover a broken down vehicle to a place of safety. Transporting a car is, therefore, illegal.
A-frames may be offered with a braking system that applies the car's brakes, these do not conform to the law as the car then becomes a "braked trailer" and has to conform to European Directives contained within the Construction and Use Regulations. It does not conform to the European Directive 71/320/EEC and amendments regarding braking requirements in any way.
The use of an A-frame for transportation is therefore illegal, however it is still OK for use to recover a vehicle to a place of safety.
The regulations go on forever, and if you REALLY want to have fun use a towing dolly, as this constitutes TWO trailers.
The rules are VERY long, complex, and, I've found, as long as you don't look susss or are blatantly taking the p*ss (towing a Shogun with a Micra) you'll usually be okay. Like many things, just be aware of the risks.
I'm tempted to buy a flat-bed now and hire it out.....