If retailers have a contract with Royal mail for the supply of stamps they pay Royal Mail at a discounted rate. Under the terms of their contract, they must sell the stamps at the rate set down by Royal Mail. That's what Big Mac is saying in his post on DrFilth's link. But that's not the whole story, because...
If retailers go into a post office and buy stamps over the counter, they can re-sell them at whatever price they like. There's no law against it, and they're within their rights to do so.