The problem all networks around the world have is that 3G data usage went through the roof with the release of the iPhone. We're not talking a gradual increase that could be planned for, it just went ballistic almost overnight.
Honestly, I'm not that convinced of that. First the iPhone 2G was released. It sold many but not nearly as many as would have sold had it of been subsidised. That wasn't even ON a 3G network. O2 had about a year to gauge the network capacity needed for their 3G network and had loads of 2G stats to work with. IPhones have been on the O2 network for nearly three years now - three years of handset growth is not "almost overnight". They just haven't bothered spending enough on their infrastructure and haven't used enough foresight. They knew perfectly well when new Apple phones were going to come out months in advance.