Next time it happens call the police and report it as " A person seemingly pointing a gun at traffic".
If its in a village with a speed limit through it or in a built up area I don't really see the problem, like has been pointed out before. Villages around the Silverstone area used to have their own speed camera peeps, trained by the police.
But if its a group pratting about on a national limit dual carriageway then they are probably breaking the law in some way, if only by causing a distraction likely to cause and accident. (which must be a law cos its the reason we don't have billboards on motorways.)