where did you find this offer?
Haven't seen that one, but I got two £15 vouchers for £25 at Argos. Which is a strong argument against everyone moaning about Apple's supposed 30% cut: If I buy software with a sticker price of £30 and pay with these vouchers, the developer gets £21. Argos got £25 minus their bank fees. I can't imagine they sell these below cost, so I'd say Apple gets £25 or less.
They just offer these from time to time; you need to check their website. And keep track of which friends and relatives like iTunes vouchers as birthday/christmas presents.