The O2 Refresh method, the HTC One M8 Metal Grey Refurb is available for £240 and it's back in stock. As usual, you pick the £38 per month tarrif which is £10 per month for the phone and £28 per month for the airtime. You cancel the airtime and pay the phone off.
You can cancel the Airtime Plan after the 14 day cooling period and pay off the Phone Plan contract. Once cancelled, you pay off the Airtime Plan used, which if cancelled on the 15th day should amount to about £14 (£28pm/2wks). Total cost = £254.
Furthermore, someone in the similar LG G3 on O2 Refresh thread said as soon as they received the phone, they paid off the Phone Plan (£240) and then cancelled the Airtime Plan only within the cooling period, thus not calculating any airtime cost.