10 steps to getting an S7 Edge for around half the retail price:
1. Click the ‘Get deal’ link
2. Ensure 'Perfectly fine' is selected
3. Choose your colour (all in stock at time of writing)
4. Go down to 'Pay your device in full', expand it
5. Choose the cheapest tariff - £15pm
6. Purchase it, delivered to your home (card address only) or an O2 store.
7. Once received, O2 will unlock it for free via My O2 > Account > Your device (https://mymobile.o2.co.uk/devicedetails) using IMEI number on the back of the phone.
8. Phone O2 on 202 (from your O2 SIM) or 0344 809 0202 and explain you've paid for the device in full and have changed your mind about the airtime and want to cancel within the 14 day cooling period. You might be charged a small fee for the number of days you had the contract (e.g. £15pm ÷ 30 days (50p) x number of days you had it before cancelling)
9. Ensure you get a cancellation confirmation (if not, call back).
10. You now have an unlocked S7 Edge in near perfect condition for around half the retail price of a new-new one (John Lewis £569.00, Samsung £599.00, Argos £639.95).
I did exactly these steps above a couple weeks ago when it was £50 more and would highly recommend. If you don't know: with O2 Refresh, the contract is split in two - device and airtime (the former you pay off, the latter you cancel). It reduced in price just yesterday when my friend was doing it - going to sell out fast I reckon!