So can I just get this straight :
If I buy an iPhone from Hong Kong and pay the whole contract in order to unlock it, will I be able to bring it back to the UK and use it on O2's network?
Does anyone have such experience?
It will be unlocked right out of the box.
You only need to pay around HK$5600 for the handset + prepayment (which will be rebated to you every month for the next 23months). The cheapest monthly plan including one additional mandatory add-ons service is HK$215 per month. For this plan, 16GB costs HK$3700 + $1700 prepayment. Also, you need to pay in advanced for the first month subscription --> Total ~HK$5600.
There are some documents to present before you can get the iPhone here:
1. HK identity card
2. Proof of HK residential address (electric bill, water bill, phone bill, etc)
3. HK credit card from which your monthly bill be charged to
Alternatively (which is easier and probably cheaper), you can buy from unofficial shops such as shops in Sintat building Mongkok. 16GB is going for ~HK$8000 as of today. 8GB is HK$1000 cheaper.