[Horace]

"likely to be useful to a person committing or preparing an act of terrorism" is incredibly broad and any photo of any Policeman could be construed as such.

This basically makes any kind of demonstration where the police may be present, pointless, since the whole of point demonstrations is to draw attention through the media.

Still, if you've done nothing wrong you've got nothing to fear..as long as you don't intend trying to prove yourself innocent with photographic evidence.