You should be fine going out with your flatmates, try and get in contact with them before getting there to get a feel for how things will be. Or look around on facebook pages and TSR to find other people and maybe you'll find a group of people to go out with.
Your age won't be an issue as there are plenty of people starting uni in their early 20s in the UK and from experience most people up to ~25 fit in, but the fresher's week you're talking about is heavily based upon drinking so I'm not sure it'll be the experience you think it is if you don't drink.