"In his attempted defence of the incident, he did however point out that riding 2 bikes or trikes side by side in breach of road traffic regulations, which was confirmed by a friend who is a competition cyclist who regularly trains on the road."
This is wrong, and the misconception is something cyclists as a group should try to address (although I don't suppose it's worth sending an extra letter to correct it in this instance).
First, and as pointed out above, you are free in law to cycle as many abreast as you desire. "Road traffic regulations" (laws) are silent on the subject, and the HC rule is advisory only, and in any case it explicitly allows for riding two abreast as quite a normal thing to do. (Gasp)
Certainly if you have not built up a long queue behind there is no reason to switch to single file (and if there is a long queue, you should pull over even if you *are* in single file).
But as a general rule there will rarely be enough space on the road to pass a cyclist safely without using the oncoming lane. This being the case, the only difference between passing a line of riders VS a double line is that the driver can pass the double line in half the time, i.e. with half the danger (they do have to move the steering wheel slightly further over though).
The only reason drivers complain about riding two abreast is that it prevents them overtaking dangerously into oncoming traffic, which is not something we should overly worry about. They have been very successful though in making even cyclists feel they should be strictly in line astern, for no good reason than their own desires.