Dpregan,
I'm not a BSAC member but I'm in a rival club organisation called the SAA.
I was encouraged to consider UK diving by a BSAC member when I was diving abroad. Buddies of mine are BSAC trained.
Most dive clubs welcome divers with PADI qualifications - it's a real myth to think that they do not. As far as I know if you want BSAC training they have a chart called the SALT agreement which is what they use as a starting point for starting/continuing BSAC training.
As for retraining - there are extra things you need to do for UK diving such as deploying a DSMB (Delayed Surface Marker Buoy) or blob as it is commonly known. BSAC use their own BSAC 88 dive tables so you'd need to know those if you want to do BSAC training.
As a PADI AOW you'd probably cross over as an Ocean Diver which limits you to 20metres as far as I know. As a PADI AOW you will be qualified to 30m but your PADI training covers you for warm water conditions - conditions in the UK are tougher - cold water and at times poor visibility so you'd need time to get used to those conditions. (I went from OW in Cancun to UK diving and the diving conditions are very different). The other thing is you might be taught how to use a drysuit - drysuits are more often the norm in UK conditions.
Complete retraining is usually down to the discretion of the Diving Officer ( Responsible for ensuring that divers are adhering to club rules/procedures) and the Training Officer (responsible for training). Most clubs will build on what you have done already and seek to expand your knowledge and won't denigrate PADI experience.
If you do want to join a club, go along to a club and have a chat.
BTW BSAC are not the only club organisation - you could also look into the SAA.
The SAA website is:
http://www.saa.org.uk/