eenu I really believe there is nothing, O2 will be able to do regarding subsidising the iPhone. They may give you some extra bolt-ons for free or some kind of tariff discount. But no matter which way you approach it you are going to have to pay £269 for the handset.
As an O2 customer who upgraded July I'm over the moon to hear that I can switch my contract to one of the iPhone contracts without penalty, seems an extremely smart move, especially for Apple who I'm sure will shift more handsets because of it.
Considering the lockdown the 1.1.1 firmware has enabled I would be a lot more hesitant to try unlocking to work with my current SIM.
Considering the facts: I'm already in contract, all providers data rates are very expensive and the unlocking complexity; an 'official' tariff seems really beneficial and the definite way to go.