This phone hasn't been out very long and it's already this cheap.
£105 with the voucher, if it works.
Next cheapest is Play.com for £149.99 already unlocked.
This Sony Ericsson C510 comes in black and features a sharp 3.2 megapixel camera with flash. The camera also features smile shutter and face detection. Send images and sounds via Bluetooth and use the handsfree facility to talk when you've got your hands full.
Even though it isn't unlocked, it can be for around £10.