Looks like this is in practice now and it sounds rediculess, check out this article.

http://www.guardian.co.uk/culture/2009/jan/21/police-form-696-garage-music

these bits stood out at being pretty stupid.

'The problem is that the Met are lone arbiters of what kind of music is high risk. Indeed, they are capable of providing a list of "risky" genres, according to David Moynihan, promoter of the club Dirty Canvas: "[I was at] a meeting with council officials and the Met, when I was involved in putting on a community festival on the Hackney/Tower Hamlets border. The police told us categorically that we weren't allowed to put on music that was 'grime, garage, rap, reggae or R&B'."'

' I was supposed to be playing in the grime room at the Music For You festival, and the police actually said, 'You can't have a grime room.'"'

WTF!?!