You are essentially doubling your money!
These can be bought off PrePayMania for £10 (which includes £5 credit). They are on t-mobile but are unlocked. So an unlocked A grade phone is worth £21 cash at Cex. You can request a prepaid envelope for free, and they send it to you, you put the phone inside it, and they pay you via a cheque or paypal. Or you can get it sorted instore if you prefer.