@GrahamS, you need to understand the Highway code, which I am sure you do, it says should not ride more than two abreast, as I put it to Carlton, His article says two abreast is ok but no more, the law says you should try and avoid riding more than two abreast.
I know it is playing with words, but in Highway Code speak if it doesn’t say MUST then it is legal, it might advise you should try and avoid but it doesn’t say you can’t. Part of the reason behind the cyclepath and hi-viz comments and the lobbying to get them removed, not illegal but only going to cause problems by there existence.
Classic example I can think of from personal experience, during the winter when I went out with the roadies, you ride two abreast as a general rule, but if you come up on other cyclists, you move out as a pair and overtake, this is fine. If riding more than two abreast was illegal, ie you must not, you would have to single out and overtake.