It would be very difficult taking photographs at a demonstration without getting pictures of police, who could then claim that you were doing it to elicit information. This might also be true on any day in London with tourists milling about with cameras and police very evidently present.
One of the aspirations of the British judicial system is that "justice should be done and seen to be done". While this is generally assumed to refer to the appearance in court, it should also apply to the actions of the police in advance of a possible appearance in court. On this basis the police should be liable to scrutiny in their work both live and on camera, still or moving.
Thinking about this further, if it is illegal to photograph a policeman because it might be eliciting information for terrorist purposes, then what about just looking at a police officer? By looking at a police officer you might be trying establish his identity from his number or perhaps judge his capabilites or state of alertness which may also be of use to a terrorist.