Apparently the bloke was wearing a Millwall T-shirt,maybe that is seen as subversive in some quarters and justification in itself.

As I understand it he was being a bit beligerent because he just wanted to get home to the hostel he was staying at,as he had finished his days work,but was getting a bit frustrated at his routes out of the area being blocked,so maybe thats why he was walking slowly etc,
Whatever the reason it is imo pointless trying to defend the plod who attacked him,there is no justification for the police to behave in this way unless the person concerned is in all probability going to cause a genuine and serious risk to the general public,which this bloke obviously wasnt.
I am not one to routinely knock the police,it is no joke doing their job in that type of situation,but SOME of them seem to lose sight of the fact that they are public servants and that is something which imo needs to be curtailed whenever possible.I think this particular cop should have the book thrown at him because he deserves it and it will help send a message to others with his attitude that it is unacceptable.