You are of course aware that the Police do that quite often, rather than go in mob handed into a demo to get people (potentially turning it violent, and still not getting the people they want), they'll have their own camera and intelligence teams to allow them to record the faces of the people breaking the law, and follow it up.
IIRC it's been part of the policing of demos, and football matches etc since the 80's, and I think was implemented in part because when they did go in in force to catch people in "relatively" peaceful demos it often caused problems (and they then got the blame for escalating the situation)
I understand what you are saying but the Police were happy to lock up EDL members at the time of the protest.


