We have the legal right to organize and participate in peaceful protest in Britain - we use the Human Rights Act (1998) to claim the right to freedom of assembly and association and the the right to freedom of expression.
If you wish to exercise this right and occupy a building, be aware that although you may be taking part in a legal activity you may run the risk of being arrested. If you are thinking of occupying a building there are several resources to help you prepare on the web. The Activist Legal Project has some useful information, as does Schnews.