Thanks again to MSE
Buy an iTunes giftcard and get the second, cheapest one half price at Clinton Cards between Fri 13 and Sun 15 August while stocks last. It's a nationwide offer, both online and instore, and you can get £15, £25 or £50 giftcards (though the latter is only in selected stores).
There's a maximum purchase of six cards per customer.
Looks like the online store have sold out of itunes cards in the offer.
Some stores have stopped the offer as they've sold their allocated amount. Probably best to call the store before going.