Don't act different around your flatmates just because they don't want to go to the pub. You don't have that in common, it doesn't mean you can't talk to them at all. Talk to them about other things besides drinking, or suggest things that you do things that aren't anything to do with the pub.
Maybe you can strike up a conversation with someone outside of your building and go around to their flat to drink and socialise, seeing as each corridor is locked. Also, as it's only freshers week, you've yet to start your course and meet people that way so don't see this as the end of your university social life.