O2 will now unlock your iPhone if your contract has expired or 
you’ve been had a Pay As You Go iPhone for 12 months.
Sine O2 lost exclusively of the iPhone to Orange and Vodafone early next year. O2 have said they will unlock the device.
However it comes with a price, customers will have to pay £15 to get the code to unlock the device.
This makes me believe there must be a similar method, as used on other phones to unlock the device via a code which will be calculated by O2.
Related:
Tesco To Sell iPhone’s Before Christmas
O2 Will Allow Unlock For UK iPhone’s
Orange Release UK iPhone Details
Reports Suggest iPhone 3G Could Be Coming To T-Mobile UK
Sam's Blog 
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