dont see why u cant get a big lan hosted at gamerbase. Manchester has 50 pcs which is ideal etc
The problem with Gamerbase is that the PCs aren't in rows of 5. IIRC there are 3 rows of 6/7 and the rest of the PCs are in columns. It's a pretty small venue and also the opening hours for it are the same as HMV. You could run a fun, 8 team one day event like they're doing but not much else.