You don't have to jailbreak any more if you are locked to O2. Google unlock iphone O2, fill out their online form, they take £15 from your credit, wait 4-5 days, put a different sim card in, link to your pc, i tunes will say, iphone succesfully unlocked. This is then an official unlock allowing you to update the software regularly. If you jailbreak it and try and update the software, it re-locks the iphone.
Danny