Hi Laura,
If you want an adventure holiday, go to a warm water resort and do a Padi O/W course. I don't think the qualification is worth much, but it quickly gets you into calm clear safe shallow water with someone to watch over you, its fun, exciting and not overly expensive.
I am of the opinion that diving should be done on a regular basis or not at all.
If you want to become an active diver, join BSAC or SAA. There is no point in joining a club, without a commitment to diving on a regular basis, and getting your own equipment. Club training for the sports diver cert is slow and if you turn up for regular weekly pool practice, you may qualify to go into the sea in three to five months time or +- 20 hours. Sea training will take about the same time. If you don't come out on a regular basis, expect the instructors to be reluctant to assess you. There is no legal contract, but the club system only survives because some of those who accepted training, help train others.
I suppose the best of both world's is to do a professional BSAC school course, then decide if you want to join the club.
It seems like half the club members got their first introduction to diving through a certificate agency like Padi.
If you want help on an article about uk diving send me a p/m, maybe I can help.
Regards,
Budgy