I've been with o2 for around 8 years. Called up yesterday for a new deal.
I asked for the s7 edge. They offered me this deal:
£15 a month (device plan)
24 month contract: 15 x 24 = £360
The airtime plan was £25 a month for 5gb but they gave me a fiver off that too.
You can pay the device plan off and cancel the airtime (I think on refresh)
Also got free Samsung Bluetooth speakers worth £99.
Good deal I think! I'll let you guys be the judge!