It's not illegal. This is from the Police National Legal Database:
"When the vehicles are attached solely by a rope or chain, the maximum distance allowed between the vehicles is 4.5 metres.
If the distance between the two vehicles exceeds 1.5 metres then the rope or chain must be made clearly visible within a reasonable distance from either side to other road users. This is usually done by tying a coloured flapping cloth in the middle of the rope or chain.
The person in the broken down vehicle must be a qualified driver as he is actually driving the vehicle. A further point is that if it is dark then the broken down vehicle must have its lights on as it would under normal circumstances."
Note: Vehicles are only to be towed on motorways, for the minimum distance necessary, to reach a place of safety where recovery may be effected by a recovery vehicle.