You can get the Nokia 2310 Red on Virgin Mobile at the Carphone Warehouse for only £4.95 when you buy £10 top-up. You could also get £5 cashback when you go through Quidco.
**This phone is locked**
Also, to bring the price down further trade-in your old brick (with manual & charger) to get a further £10 off making this phone technically free (provided you get your quidco).
To get this phone unlocked you (or someone you know) must have spent £30 on credit to request the unlock code for free from Virgin Mobile.