So far as I am aware (being an ex RAC Patrol) towing with a rope is a short distance only method. Certainly only to the next available junction on a motorway!
On other types of road I don't think you could be prosecuted unless the vehicle you are towing is not taxed or insured; has bald tyres or does not meet the requirements of 'construction & use' regulations; is being driven (steered) by someone not holding an appropriate licence or is being towed in a dangerous manner.
The RAC (again, as far as I know) still use a tow strap when other methods are unavailable but only for short distances.
I can't see how the use of a strap/rope is so different from the straight bar used by the RAC/AA lads.
a Friend of mine is a Copper so I'll ask him to find out for me.
The main thing to remember is:
Even if you are using a 'Spec lift', the vehicle MUST be legal. This includes tyres, lighting (or trailer board), Road Fund Licence AND INSURANCE. Just because you are towing the vehicle doesn't mean it's covered by your insurance unless it's being steered by a named driver on your policy. If you are towing the vehicle the responsibility for it's legality is YOURS!
This is my understanding of the regulations but I may be a little out of date.
Hope this helps.
Paul.