I did a lot of research into this last year when I was considering moving a broken down 3.5 ton horse box from Cheshire to Yorkshire late one night with my Discovery. The law isn't clear but it is enforceable. Some of les garcons bleu don't know the law, some know it inside out and backwards... This is my interpretation - which may be wrong - but it was enough to put me off the idea.
- "A-frames" and "Dolly's" are classed as trailers and therefore trailer laws apply. That means a full lighting board, number plate of the towing vehicle and if the gross kerb weight of the vehicle + towing device exceeds 750Kg then every wheel of the "trailer" that is in contact with the road must be braked. That means all four wheels of the car if you are using an a-frame OR the dolly wheels plus the two rear wheels of the car if a dolly is being used. NOTE - gross weight means what is stamped on the car and not what it actually is - so if you have a Smart Car when the engine, gearbox and interior removed which weighs 550 Kg but is stamped at 751 Kg - needs to be braked. The exception to this is where the towing vehicle is registered as a breakdown/recovery vehicle.- Hence AA and RAC vans CAN use them (and pay less tax...) "Braked" means operated by an overrun coupling.
- Rigid tow poles are not classed as trailers and are a legal means of moving vehicles, but the vehicle being towed must be road legal (road worthy, tax, MOT, insurance) and the maximum towing weight of the towing vehicle must not be exceeded.
- All of the above goes out of the window if you are towing (rope, pole, dolly, a-frame) a broken down vehicle from the highway to a place of safety (i.e. next services on the motorway) but you could be prosecuted for other offences if doing so creates a danger (i.e. towing a Disco with a Metro?).
You often see Smart cars, mini's etc. on A-frames and dollies at the back of motorhomes - they are not legal.
That said I was on the M62 in East Yorkshire recently and I saw a Police T5 waft pass a M reg Vauxhall Cavalier that was towing a trailer with a Renault Leguna estate on it at around 70 - way over weight and over speed but didn't do anything about it. Bear in mind they can force you to abandon the trailer/car and arrange recovery.
If anyone has a different interpretation of the law I would genuinely like to hear it.
Richard