Many, many points I'd like to make on this but for clarification on the events of the day:
As soon as the demo entered parliament square police lines formed on every exit, however most of these lines remained "pourous" until just before 4pm when for no apparent reason they got shut off. To be fair there had been a bit of conflict by this point but nothing worth getting excited about - nothing that left anything more than a few bruises on either side.
As usual as soon as the kettle was in place those that wanted to kick off felt they had carte blanche to do so (and I refer to both protesters and coppers with that) and those protesters that didn't necessarily turn up seeking conflict felt they were justified in using vandalism to voice their objection and violence to try and rectify the situation. There was about a 15 minute period on Whitehall just after seven where it was something of a free for all. The police seemed unsure of whether to fall back or hold firm under mounting pressure and as a result some of them stayed and got, understandably, very fecking violent in a pretty hostile situation and the rest ran back. They eventually regrouped and took the road back pretty easily.
I was in the kettle until half seven when I was let out in one corner where FIT were insisting everyone who wanted to leave be filmed and photographed (despite no legal justification for doing so). I know people who didn't get off Westminster bridge til nearer midnight. It seems the police did a bloody good job of letting everyone who was a bit more "spikey" out pretty early, either through their 2 at a time dispersal system or by just having their lines broken, and containing groups of largely peaceful protesters until much later, which seems an odd tactic.
I will say I was impressed at how quickly a lot of the kids had learned simple disorder tactics. Keeping in mind the vast majority of these students had never been on a protest until a month or so back the knowledge of line breaking, effective missiles and things like reinforced banners was something many thousands had obviously gone out of their way to learn. Make no mistake, the "small minority" of people who just turned up for a ruck doubtlessly existed but it was in fact the vast majority of attendees who were willing to take direct action and use force to make their point and then, later, resist the police.