Following on from my post back around Page 59, I received my 16gb Space Grey 5S from O2 this morning.
On Saturday, I phoned O2 and signed up for the Refresh tariff of the following:
Airtime Plan: £17 per month
Phone Plan: £169.99 up front, and then 24 months x £15
Received the phone today and I called O2 and paid off the Phone plan (24 x £15 = £360), and cancelled the Airtime contract too. There is no termination charge for this. While on the phone to them, O2 have said they will unlock 5S's as long as the Phone plan is paid off, and not at any time during the contract.
So, here I am, in total I've paid £529.99 for a phone that costs £549 from Apple. A few people earlier tried to say I'd have to pay the £17 x 24 months as well to cancel it, but O2 show it on their website that it's not the case and there is no fee to cancel it.
If you really want the 5S, I'd go down the same route I done, especially if you were ready to pay for the phone outright. I didn't even have to pay for the airtime charge.