In reply to Tyler:
LCC are the biggest chain with 8 locations and an emphasis on more central locations (bouldering only but Harrowall has autobelays), but one of my friends estimates there are 70 walls inside the M25 (including ones that are part of schools, leisure centres, gyms, youth clubs etc).
LCC are one of the more commercial and social media savvy brands as well. Whereas, for example, notwithstanding the fact the bouldering is minimal and they're closing for refurbishment, hardly anyone seems to know about West1 at Seymour Leisure Centre - but that's very local for people in parts of the West End.
I forgot about City Bouldering Aldgate! An easy trip on the Circle / District line or Central line.