The Carphone Warehouse have the LG "Chocolate" Phone, or LG KG800, in Black on T-Mobile and Vodafone pay as you go. On these two networks it's just £34.95 when you buy £10 of airtime (a package price of £44.95). Next day delivery is free. It's available on other networks, but prices vary. If you wish, you can purchase via QuidCo.com for £5 in cashback.
The LG KG800 is a smooth, slider phone and looks like a chocolate bar. The external keys are actually invisible, they light up red when ready to use. It has 128 MB internal memory with an MP3 player and MPEG4 video player. The KG800 also has Bluetooth and a 1.3 megapixel camera with flash and video record.