That's the policy at my job. Stuff goes down in price all the time, and without a receipt we can only exchange the goods at the current selling price. Without proof of purchase, the store can't know if you paid the reduced price or full. It wasn't a mistake by the shop staff, but the system updating a reduced price. sucks, but there isn't much that can't be done if you're lost the receipt. That they even gave you money back at all is more than they're legally obliged to. Writing to head office will only get you told the policy parrot-fashion. TBH I'm suprised that so many people are suggesting it when River Island have followed policy that many, many high street shops have to the letter.
I'm really pants at keeping receipts. I really should given that I know many shops have a similar policy to that.
EDIT: Just reread and noticed that they weren't in the sale. How odd. Scrap what I said and complain, heh.