Hi Paul, POAH is correct actually, there's quite a lot of info on this already on the forum - however be prepared to see an awful lot of uninformed comments from people who have no direct knowledge of (nor experience of) the organisations in question. The question of qualifications can become heated, even when qualified contributors are simply trying to point out that the value in gaining accreditation lies in the journey toward self-improvement rather than the certification itself. In that regard qualifications are not necessary at all, but they can be a great learning aid and at times will open the odd door.
In any case, there are four 'main' well-known long established institutions. Each institution has a slightly different flavour and will attract a slightly different membership. So it really depends on what kind of photography you do. The BIPP (British Institute of Professional Photography) is quite prestigious and has a strong fine art bias - they will accept part-time professionals and there is a probationary period during which you must work towards the first rung of the qualification ladder. The MPA (Master Photographers Association) tends to concentrate on wedding and portrait photography and it is a requirement that you are a full-time professional and you must gain qualified status in order to progress your membership. Both the RPS (Royal Photographic Society) and The Societies (SWPP and BPPA) will accept unqualified members and you do not need to be a working professional, which makes these organisations particularly popular and accessible for newcomers. The RPS has a strong following with camera clubs and a bias towards landscape and documentary nature photography. As for whether there are regional meetings in your locality I cannot say. If you're new to the industry and lean towards wedding or portraiture work The Societies are a good choice - there are training seminars and workshops throughout the year across the country and the quality of the training is top notch - as Steve has said the yearly Convention is fantastic and members receive substantial discounts off any tickets, there is also a very active forum with established working professionals on hand to help and support those who wish to improve their photography.
All four of these institutions have the same qualification structure: Licentiate, Associate, and Fellow. The organizations often share judges between themselves and so the standard of qualification is quite consistent, although the RPS are unique in that they require only 10 images at Licentiate level whereas the others require 20 (that figure rises at the BIPP where you will be expected to show a fuller body of work as you go forward).
I am a qualified member of the Societies, the BIPP and the RPS. The RPS has no particular relevance to the work I do, I joined simply because I enjoy all sorts of different genres of photography and the RPS has a number of special interest groups. The Societies are very active and very sociable, you will make friends and you will reap what you have put in as has already been said. The BIPP encourage their members to progress and develop their style and this is therefore a very good organisation for the more ambitious creative photographers.
At the end of the day it really depends on what you want to get out of any membership, if you want to make friends and contacts you will need to make an effort to get yourself known and to take part in whatever is on offer. If you want to improve your photography then the training opportunities are there - as is good quality mentoring.
But they are all about £100 pa for membership