Via A Very British Dude:
Ok, so the film-maker is a bit annoying, but he's making a good point. He's filming in his garden, and yet the Police act as though they are the law, declaring that an offence is taking place (although unable to specify the legal source of this offence).
Aside from the obvious question of "where the hell did they learn that particular law?" this is an example of the police being a law unto themselves.
I know that the job can be tough, that inner-city violent crime is a growing problem, but how exactly can their actions here be justified, other than saying that the pair of them were on an ego trip.