dude, you can do what you want in your room. you're paying for it, for gods sake. the university says you can't have pets there, and the law says you can't have anything illegal, but apart from that, you can do whatever the hell you want. think of it as your own mini house that you're borrowing from the university. they have no right to say what you can and cannot do in that room, so long as it's legal.
bless you, i think you're going to find england a lot more relaxed than what you're used to. nobody really cares what you do in your room with your friends or anyone else.
(I SAY THIS IS A VERY UNOFFICIAL CAPACITY) You'll also probably find people are very relaxed about smoking various not-exactly-legal substances.