Diving in the UK
Over here in the UK you have 3 agencies to learn to dive with.
PADI, BSAC (British Sub Aqua Club) and SAA (Sub Aqua Association).
I am with BSAC.
For PADI certification your friend just needs to find the local PADI school and book on a course. I would expect that the prices are pretty much fixed.
BSAC and SAA work differently, they are club based. You join a local diving club, pay membership fees to the club, pool fees and also pay for your training packs. Training is provided by the clubs instructors free of charge. Most clubs meet at a local pool once a week and do open water dives at weekends etc.
Now club fees will vary from club to club. BSAC membership is I think £40 per year. My club membership and pool fees are £120 per year and I think my training pack was £15 so my first grade has cost me £175. However I am now doing my Sports Diver qualification and all it has cost is £15 for the books as my membership fees etc are not due until February.
I dont know how this compares to PADI but would think BSAC and SAA are cheaper.
At the end of the day it all depends on how often you want to dive. If your friend just wants a certification to dive on holiday then PADI may be the easiest way to go. If they want to dive regularly in the UK then i would suggest a BSAC or SAA club.