In reply to James Malloch:
You can join a BSAC club and get taught quite cheap. It works on a volenteer basis with people teaching for free. Most clubs have kit to hire and you can gradually buy your own. Kinda like joining a local climbing club.
PADI is a commercial scheme sold through shops. Generally quicker, but more expensive. Kinda like going on a climbing course at PYB or similar.
Rapidly growing in the UK is GUE. They mainly teach advanced helium and cave diving courses, but have arguably the best basic course in the world (GUE Rec 1). The emphasis is about real skill in the water with bouyancy and movement control, not just the usual mask clearing etc. Kinda like having Neil Gresham teaching you to climb on a 1-1 basis for 10days.
I know John Kendall, GUE instructor will be doing a meet and greet session at Diving Lesuire in Leeds on 7th Dec. Might be worth a chat if you're tempted by it.
Give me a shout if you need any more info.