This is what the CPS guidance says:
There must be a person within the sight or hearing of the suspect who is likely to be caused harassment, alarm or distress by the conduct in question. A police officer may be such a person, but remember that this is a question of fact to be decided in each case by the magistrates. In determining this, the magistrates may take into account the familiarity which police officers have with the words and conduct typically seen in incidents of disorderly conduct. (DPP v Orum  Crim LR 848)
Seems fair enough to me.