Don't know where in the UK you are, but I just got back from a fun saturday night with two women from AA. One is 21 and has been sober 2.5 years. I am 28 and just starting. The other is 37 and has been sober 9 years. In the young persons meeting I go to, there are a few under the age of 25 and sober a few years. So they do exist.
I understand about the pub/drinking culture here, especially at that age. I can imagine it must seem hard and can't really offer any advice. But hang in there, there are other sober people your age, and the ones I have met so far are really fantastic.