I bought it in Guildford. First I asked them whether it available on 'pay as you go'. You can see the advertisement on the material in a vodafone store for the payg price. Then I told them I would go back to China in few days, so I needed to unlock the phone, and I was willing to pay the unlock fee. That's all.
Try a vodafone store but not a phones4u or carphone warehouse.
Try to buy a payg phone but not a headset only.
Try unlock the phone in a vodafone store, as far as I know, nobody else can unlock it in China town.