you will see it mentioned in this debate on the transport of dangerous goods by road but apparently it isn't illegal.
this is taken from thje debate, which was in 1998, couldn't find anything more up to date so assume there is nothing.
Another problem which I have encountered on numerous occasions is the incidence of heavy duty goods vehicles travelling in convoy. This is becoming a common feature on our roads, as I discovered on my frequent trips to the west and south. For reasons best known to themselves, lorry drivers appear to enjoy travelling in convoys probably because they do not have to contend with other road users. Other road users cannot cope when confronted with a convoy of two, three and often four lorries which sometimes travel at speeds in excess of the local speed limit. Such convoys prevent them from passing the lorries, as they would do if they encountered them singly. This type of practice must be condemned and enforcement of the rules must be rigorous. I hope the Minister of State will tackle this problem.