The Serious Organised Crime and Police Act 2005 creates 'designated areas' where  demonstrations or protests are suspended unless prior permission from the Secretary of State has been obtained.

This should have no bearing on photography but in practice it does not work out like that. Anyone photographing unauthorised protest within the designated area of 1km from Parliament, Westminster Bridge, New Scotland Yard is liable to be regarded as a protester rather than a photographer and detained or arrested, and that includes professionals reporting on events.