This video (Hat Tip to Old Holborn) is yet another demonstration of the abuse of Police power.
Things to remember: First they will try and stop you recording the encounter. This is to attempt to pervert the course of justice.
Second, they will attempt to pursuade you that what they are doing is within the law, which in this instance, is not.
Third, they undermine public faith in the Police Force.
Oh, and for what it's worth, the guy in the film with the megaphone is a dick head. But it is not yet illegal to be a dick head or act like one.