good video, i take it this was during the 'riots' in london a while back?
That was my first though also, that it was a part of the riots; but believe it or not, this took place about 5 months prior to the london riots. I found that what's happening in the video is actually called the "London anti-cuts protest." AKA: "The March for the Alternative."
All the riots and clashing in the video were essentially done by other independent protest groups who strayed away from where the main gathering was taking place.
Apparently 201 people were arrested, and 66 were injured, including 31 police officers.