Vodafone is currently selling the Motorola Moto E on PAYG for £45. Choose to “Keep your existing Vodafone SIM” to avoid having to buy a top-up. If the site hangs when you add the phone to your basket, delete the last portion of the URL, leaving just http://shop.vodafone.co.uk/shop.
Motorola Moto E specs include a 4.3in display (540 x 960), Qualcomm Snapdragon 200 processor, 1GB of RAM, and a 5MP rear camera.
(Thanks to seattle at HUKD)