Quick google.
Name: mobile web
APN: mobile.o2.co.uk
Proxy: *LEAVE BLANK*
Port: *LEAVE BLANK*
Username: o2web
Password: password
Server: *LEAVE BLANK*
MMSC: *LEAVE BLANK*
MMS Proxy: *LEAVE BLANK*
MMS Port: *LEAVE BLANK*
MCC: 234
MNC: 10
APN Type: default
A lot of people in the iPhone thread saying it's a problem on o2's end atm. Might be that mate.