Use Code TR756S to give you £75 off the £150 upfront cost.

iPhone 6S 64GB, Any Colour
Vodafone 4G
Wifi Calling
£75 Off TR756S (Normally £150 upfront)
Unlimited calls and texts
6GB data (Unlimited for the first 2 months).
£31 P/Month

Links to different colours in comment #2.

This is on par with the very successful deal that was on offer around Black Friday, 3 months ago or so, but this time with 6GB of data on Vodafone UK.
http://www.hotukdeals.com/deals/iphone-6s-64gb-o2-4g-100-off-using-code-blacknov15-unlimited-calls-texts-3gb-data-2319260

I ordered this yesterday, was delivered today, and from my Vodafone account, it appears you actually get Unlimited Data for the first 2 months, and 7GB of Data ongoing.

Mobiles.co.uk Local Rate Customer Service: 01509 615474

£10 TopCashBack / Quidco too.

£34.13 effective monthly cost.
£33.71 effective monthly cost with TCB.Quidco.

This is by far the best deal on offer at the moment.
https://www.mobilephonechecker.co.uk/Apple/iPhone+6s+64GB.htm

Also available on the 128GB for £125 upfront. Use code 125OFF6S for this one (choose the same £31 Vodafone tariff with £250 upfront).
http://www.hotukdeals.com/deals/iphone-6s-128gb-6gb-data-125-upfront-31-a-month-vodafone-869-mobiles-co-uk-2388480

£25 extra with Mobiles.co.uk and Mazuma mobile when trading in an old phone.
http://mazuma.mobiles.co.uk/

another option

if you want a a different network instead of vodafone for the same price you can go onto O2 but with 3gb data
https://www.secure-mobiles.com/checkout/checkout.php
- BoGoF