In reply to RoryT:
Your first port of call, as someone has already said, will be to look into local climbing clubs ( not all take under 18s though ). If there is a local climbing wall near you go there to learn and practice and also make contacts - many walls have a noticeboard to help people meet up.
You could consider a climbing course - I went to Plas Y Brenin for a rock climbing course when I was 16 and learned loads from it. Check out the Conville course ( I supsect it is for over 18s ).
The British Schools Exploring Society used to run expeditions. You might also consider Ten Tors, if you are not too far from the Southwest, or D. of E. Both of these will set you up for some good skills required in mountaineering.
Enjoy!!