I suspect if he took it far enough he could have that fine rescinded.
It is not illegal to tell someone to 'piss off'.
Under section 5 of the Public Order Act 1986:
A person is guilty of an offence if he ... uses threatening words or behaviour, or disorderly behaviour ... within the hearing or sight of a person likely to be caused harassment, alarm or distress thereby.
If the police officer were likely to feel harrassed, alarmed or distressed by his use of 'piss off', then it is an offence.
They certainly weren't harrassed, and very unlikely they would have been alarmed or distressed either.
He, on the other hand, certainly was harrassed by the police officers, and I think their behaviour could reasonably be construed as threatening. If I were him I would be making a formal complaint.