it depends how things are done in your town/city - generally people will be happy to buy you alcohol so you can drink with them and going to house parties isn't going to be a problem, some clubs won't start IDing people til 9pm so if you go early you can get in without ID and some SUs will let you in with your student card (which may or may not have your age on it) but generally you're going to find it hard to go clubbing if you're underage (unless you have a good fake ID?)
it's going to be much harder if you're underage to make friends, consider joining a lot of societies... definitely tell your flatmates upfront! I lived with a 17 y/o and all had to leave a club after paying as he didn't tell us he couldn't go in!