C&P from the FAQ page
Why can't I 'unlock' my iPhone?
We may not be able to unlock your iPhone for a variety of reasons. Here's a few common ones:
You may have not typed in the correct IMEI number (found by keying *#06# on your iPhone) and the correct mobile phone number. If these don't match the numbers we have on our system, then the iPhone can't be unlocked.
Your iPhone must have been purchased from an O2, Carphone Warehouse or Apple UK store for it to be unlocked. If this isn't the case, then you need to go back to the place you purchased your iPhone from to get it unlocked.
Your mobile number may have been barred on all networks if you've reported it lost or stolen. If your iPhone is no longer lost, please contact our customer service team.
We can't locate your iPhone on the O2 network. This may happen if you haven't 'synced' your iPhone with iTunes.
If you're a Pay & Go customer, the iPhone must have been active on our network for a minimum of 12 months.
If you're a Pay & Go customer, you may not have had enough airtime credit on your account to pay the £14.69 fee required to have your iPhone unlocked.
Note: if we're not able to unlock your iPhone, then the £14.69 fee will be automatically refunded for our Pay & Go customers.
As a fellow Adelaidien in London i'd gladly help out if I can in anyway Billo (5 year lurker, first time poster)