Well, despite it being all over the news... Tottenham is an area with a long history of racial tension. Sort of like Bed-Stuy in New York or Oakland, CA. Unemployment is high and the humiliations of poverty abundant, police search and seize pedestrians liberally, and to top it off, they shot a black guy to death last week. This is no way excuses the brutality of what's going on there, but that's why its happening.
It seems more economic than racial. Most of the pictures I've seen of rioters seem to be a fairly even mix of black, white and middle eastern looking people.