Being vegetarian and a non-drinker ought not harm your social life. Restaurants in the UK pretty much all cater for vegetarians, and wherever you go for drinks you can always drink cola. The question really is just how to start building yourself a social life. You do this by forcing yourself out of your comfort zone and talking to people. It can be really difficult depending on how against your personality it is, but you can force yourself if you set your mind to it.
Go out to some societies, make a few friends, then organise a social event and invite people. Go out, show people a bit of fun. Be fun and people will want to be around you. I'm not saying it's easy, but that's the answer to your question.