BSAC is an excellent system to learn to dive, but most of the training and diving is centred around hundreds of small BSAC clubs which are nearly all UK based, most training is done within your particular club, by other club members, a lot of clubs have their own boats, trips are organised by the club for its members, some trips may be based around training, some trips may be organised to suit only experienced divers, some may be to overseas destinations. Each club has its own requirements based on its particular members.
As I said earlier if you wish to join such a club and dive regularly in the UK, BSAC is by far your best option. Also I would recommend doing your training within your selected club so you will learn as you dive and learn about your club and be involved with the other members, rather than simply paying to do a course. If you find a good club it will be great fun. I personally dived with a BSAC branch for many years before moving to Thailand and thorougly enjoyed it. Had many excelent trips diving over the years, also a good social aspect to the club.
If you do not wish to become involved with such a set up and simply want to do a course and then dive on a ad hoc basis. PADI training in my mind is far more suitable.