S6 via the O2 refresh deal that's going about, pay the phone off £240, request unlock code, cancel contract and you got a GS6 for £259(£19 up front), also cancelled direct debit, got one myself its a steal for a Galaxy S6. It only had 4-5 O2 apps on which you can get rid of but I flashed an official BTU UK rom on.
As you in the 14 days grace period nothing they can do about it. I had mine unlocked and paid off the same day and contract cancelled. You just need to make sure you don't get a numpty when cancelling contract.