This is quite extraordinary.
Are the police are making up the law as they go along?
It sounds like someone in the police sent two officers out to infringe freedom of speech. That can only be because they are ignorant of the law, or because they have an agenda to limit freedom of speech. The fact that this is not the first time that this has happened, and that this is not the only police force in England and Wales where it has happened, suggests that this is coming from the top. In other words, ACPO.
(Thank you very much to Cranmer for this)