You're right - they're under no obligation to sell if they don't want to, but if they're going to put a restriction on the number of products that they're prepared to sell you, then they're supposed to make that clear in signage to the customer. So if they want to do it, then they need to put a sign up on the gift cards saying that there's a limit. If they don't, then I would argue that it's potentially a misleading omission under the Consumer Protection From Unfair Trading Regulations 2008 and while I would challenge it at the till, if they insisted on not selling I wouldn't bother getting into a fight about it. I'd simply leave without my purchase and report the store to my local Trading Standards office so they can go in and have the discussion with the manager.

1F