Well, that didn't take long. It's a matter of hours since the UK telecoms regulator Ofcom brought in new rules to protect mobile customers from mid-contract tariff increases, and already one network has ducked them by changing its T&Cs. As of today, any customer who signs up with O2 must explicitly agree that their tariff will go up each year in line with inflation, starting with a 2.7 percent increase on March 1st. By contrast, customers who had signed up before today had only been told that prices might go up with inflation. Why is this so ironic? Because, indirectly, Ofcom's involvement seems to have turned a possible price increase into a definite, contractual one, whereas the original intention was to ensure that "fixed means fixed." Anyhow, in O2's defense, it hasn't breached Ofcom's code, and other carriers may well be planning to do the exact same thing.
Update: Unsurprisingly, other carriers are now making their feelings on the news public. In a statement, Three UK has announced that it'll follow the spirit of Ofcom's code, and won't push your prices up part-way through your contract.