o2 have probably realised that they don't need to have long opening hours in order to attract business. If you look at o2's customer base and profits I'd say they're doing quite well without 24 hour customer services.
Besides, the number of people who actually call customer service lines late at night is very small and often the call could have waited until the next day. I've done the shifts up to close and as it gets towards the end of the night it really dies down, especially Friday, Saturday and Sunday nights.
o2 broadband may be 24 hours but I'd imagine it's because 2nd line have to be there anyway so they may aswell answer the phone if it rings.
