Just for precision:-
A person is guilty of an offence if he—
(a)uses towards another person threatening, abusive or insulting words or behaviour, or
(b)distributes or displays to another person any writing, sign or other visible representation which is threatening, abusive or insulting,
with intent to cause that person to believe that immediate unlawful violence will be used against him or another by any person, or to provoke the immediate use of unlawful violence by that person or another, or whereby that person is likely to believe that such violence will be used or it is likely that such violence will be provoked.
Or to put it another way - you won't be arrested for simply saying "fuck" in public. You won't normally even be arrested if you just tell a copper to "fuck off" - unless he feels threatened. If a cop arrested someone every time he was told to fuck off, he'd literally have no time to do anything else.
Frankly, why should coppers - or anyone else in public service - have to put up with "threatening, abusive or insulting words or behaviour" when they are just doing their job?