I'm not an authority on UK diving but the way I see it, there are two popular agencies in the UK, the BSAC and PADI.
the BSAC is based on CMAS standards and involves a "club" system. You generally pay dues to the the club but training is either included in the dues or at least the lay-out is easier to digest (from what I've heard--this is 2nd hand information). The advantages are numerous. Having built in dive buddies, extensive local knowledge and training that is tailored to conditions in the UK. The BSAC is also a widely respected agency and cross over is relatively painless. The strength is also its downside. Training can get dragged out. Some clubs will exercise considerable influence over your development as a diver (for better or worse) and due to the (natural) arrogance of the BSAC with respect to PADI may close some options to you unless you have the strength of personality to tell them where they can stick it.
PADI, in contrast, does not have a club system, which I personally find to be an advantage. Training is .... compact (ie. tempo is high), which leaves some of the responsibility for perfecting skills in the hands of the diver .... and because PADI was more or less developed in California and/or the Caymen islands, the approach is not applicable to UK diving unless you find an instructor with enough "vision" to make the material work in the local context. My experience is that these instructors are out there in large numbers but the PADI standards are more generic and do not address local UK conditions as well as BSAC standards do. The upside to this that the you are left with a lot of freedom to do and/or develop as you wish.
Making contact either way just involves making a few phone calls.
Does that help?
R..