I've been looking to upgrade from my trusty s6 for awhile - and have gone for this deal as the best I've seen recently!
This is for the Samsung Galaxy s7 edge on EE - with 5gb of data and unlimited texts and calls for £27.99 a month. It's £25 upfront but you can use the voucher VC15OFF to take it down to £10. You can also get decent quidco (mine tracked at £20).
In addition at the moment you can get a free Samsung Gear Fit 2 sports watch with purchases of an s7 or s7 edge - rrp £179.99. This offer is available with lots of retailers by redemption, so you may be able to find a better deal to suit you and still get it free!
As mentioned by another member, Mobiles.co.uk are unlikely to accept a cashback claim if you use the voucher. They state that in their terms for the voucher. It's a choice of getting £15 off straight away or £35 after a few months, not both.