Edward is right about the revolutionary angle - if you think about the language used, the illegal act is "marching on parliament". The origin of that phrase is military campaigning, where the idea of marching on a place implied an intention to storm it by force.
It's a delicate balance between maintaining the right to protest in sight of your elected representatives, and ensuring that those representatives don't feel physically threatened.
As it happens, the usual place for protests of this sort is Trafalgar Square, not Parliament Square, which is usually considered sufficiently "in the heart of London" for the early evening news editors. Interestingly, both squares are controlled not by Westminster Council (as is the case for most other public spaces in the area), but the Mayor (whose office happens to be well-connected to the NUS).
Sorry about the late receipt of the email. I don't send out College-wide emails directly. They are sent out for me by ICT only after approval from the Deputy Rector. So my control over their timing can be limited.