There's absolutely nothing in law which specifically states that it's illegal to use the F-word. What the Public Order Act 1986 actually says is this:
"A person is guilty of an offence if he�
(a) uses threatening, abusive or insulting words or behaviour, or disorderly behaviour, or
(b) displays any writing, sign or other visible representation which is threatening, abusive or insulting,
within the hearing or sight of a person likely to be caused harassment, alarm or distress thereby".
The Public Order Act also states:
" It is a defence for the accused to prove that he had no reason to believe that there was any person within hearing or sight who was likely to be caused harassment, alarm or distress".
I refuse to accept that your average plod would be "likely to be caused harassment, alarm or distress" simply because you (allegedly) said "For f*ck's sake". If a court took the same view it would have to rule that it was perfectly legal to (allegedly) use such words.
Further, The Public Order Act states:
" It is a defence for the accused to prove . . . that his conduct was reasonable". Under the circumstances, your (alleged) words sound perfectly reasonable to me!
The police frequently mis-use the Public Order Act just to bully people, or simply to improve their crime statistics. They rely on the fact that most people either don't know the law or won't bother to challenge the penalty notice. Go to court and fight them!
Chris