Free 16GB iPhone 6S is now possible for purchase in O2's £39 per month deal.
Beside this, you are getting 5GB of data which is quite okay, if all you need your internet connection for is to check Facebook or post a picture on Instagram and in addition you are getting unlimited calls and texts.
If you are intrested in more capable iPhone 6S, you can get 64GB in the same price contract as 16GB version, but you just have to pay £60 for device on start, which is really impressive, if not the best deal in UK for iPhone lovers.