Seems to be widely available on PAYG Vodafone for under £100.00 from Tesco, Argos, Sainsburys but cannot find it on T-mobile or sim unlocked for anywhere near the £100 mark. According to Hotukdeals these were in-store at Tescos for £80.00 last week so you might be lucky......
1 guy on ebay is selling unlock codes for £5.49 if needed.