Er . .yes. . and . . er . . .no.
Much of motoring legislation is based upon 'general', rather than 'specific' rules. For example there's no law which specifically states that you can't drive down the right-hand-side of the road. If the police see you doing so, they have to decide which of the 'general' laws you've broken. (i.e. they'll consider charging you with 'driving without due care and attention', 'careless driving' or 'dangerous driving'. They can't charge you with 'driving on the wrong side of the road' because there's no such specific offence).
Similarly, there's no specific offence of 'towing a vehicle other than with a tow bar'. However, there's now far greater awareness than there used to be that using a tow rope can be unsafe. The (recently revised) Highway Code restricts itself to this statement:
"In the event of a breakdown, be aware that towing a vehicle on a tow rope is potentially dangerous. You should consider professional recovery."
http://www.direct.gov.uk/en/TravelAndTransport /Highwaycode/DG_069855
If a police officer sees you towing another vehicle with a tow rope he has to ask himself whether a court would be likely to rule that the procedure was unsafe. If so, you'll probably end up with a fine and penalty points for contravening the relevant section of the Road Traffic Acts which deals with failure to have proper control of a vehicle.
Chris