I have a £15 iTunes gift voucher that I got for christmas, but just haven't got round to using so I thought I'd sell it on. My asking price is £11, which I think is a fair price, earning you £4 worth of free downloads. I also checked ebay and that seems to be the average selling price (e.g item number: 140203838433)
I also found a thread on here where one also sold for £11 : http://www.hotukdeals.com/item/141730/fs-15-itunes-card-sold/
I can take payment by paypal, and I will then PM or email you the code once the transaction is complete.