Similar to AT&T in the US, O2 has now put a data limit on pay monthly users. O2 had already put a 750MB limit of Pay As You Go users but have now changed there unlimited plans to have limited data caps. The cost for additional data isn’t that steap and for those who need it shouldn’t be that much of an issue but it opens up a competitive advantage to any of the multiple UK carriers who offer true unlimited data.
Sam's Blog 