Alcatel Onetouch will be selling the Pixi 3 (3.5) - confusing name alert! - in Sainsbury's for just £20.
Naturally, you don't expect the latest specs on a £20 - I have to say that again £20! - smartphone, but you can still message, Facebook, look up stuff, and find places, as well as take basic photos. It can even take memory cards. It uses Android KitKat, so it's not some bizarre OS, and of course you can get access to Google's Play Store. You can pick one up online from here.