The early part of the video which is not on this video shows mortars hitting the base, from where? who knows? So I'm sure the British were pissed off when they ran out after the crowd.
This video shows a bunch of young people gathered about kind of like young people in the west do at civil protest ( they throw rocks and shit too sometimes), but the reaction of the British was over the top, if a kid throws rocks, adults don't get the right to randomly grab the teenagers and kick the crap out of them. Do they know if those particular kids were responsible for anything, no, they just wanted revenge for being attacked. Totally reprehensible behaviour which does not help the situation in Iraq.