.... Towing on a motorway incidentally, contrary to popular belief, is not illegal either. ...........
As I understand it .... a car can only be towed by a rope on a motor to the next exit .... certainly not entering a motorway on a rope.
There is nothing illegal about towing a vehicle on a motorway with a rope, whether it be to remove a stricken vehicle from the hard shoulder, or joining and travelling several junctions to tow a vehicle via the motorway to a destination, as long the tow rope is of the correct length, is marked to indicate the presence of the rope to other motorists, the stricken car is road legal, you preferably have an ON TOW sign in addition to the reg of the towing vehicle displayed, and the likelihood of your actions isn't going to cause danger to other road users.
Having said that, common sense should dictate that it IS a dangerous thing to do, as to the probable speed you would be towing at relative to the speed of other vehicles alone would make you a hazard in your own right. The likelihood of an issue if the tow rope becomes slack, or the towed vehicle runs over the rope, especially with those inexperienced in towing with a rope whether that be the tower or towee(?!?) are very high, and would be likely to land you in a lot more bother than the cost of a professional recovery, or at least the cost of a solid bar or 'A' frame which are infinitely safer, especially on a motorway.
In short, its not illegal, but certainly not recommended.
It's all common sense really, or should be.