The sort code covers all Abbey branches - I have 3 accounts with Abbey, opened at different branches in England & Scotland and the all have the same sort code. The Abbey is now using sort code 090127 (I suppose they ran out of numbers under 090126??).
If you are filling out a mandate either put down the address of the branch you are with or the London Address, as shown on the Cheque Books.;-
p.o. box 382, prescot street, london e1 8rp
You will find that most companies will accept either.
p.s. the Halifax & Coop also have 'centralised banking'