there's a difference, legally, between mistake and misrepresentation. I don't know the law in the UK, but in the U.S. you could never nail anyone with legal proceedings for false advertising or misrepresentation of a product unless it was being misrepresneted at the time of the offer. I'd be willing to bet that there are all kinds of contingency boilerplate info written into pre-sale order disclaimers, not to mention the distributors/producing company's contract with vendors like Amazon.
You're right, it stinks, and they shouldn't have put it on the market until they knew that contingency was no longer possible. However, if they are refunding your money then that is about all they could ask given the circumstances. Besides, strictly speaking, if they added stuff to the Box Set, then if one wanted to make them "stick to their deal" then they'd have to send you what was originally going to be in the Box Set, not what is actually in it. Add to that the fact that that product technically doesn't exist, and you have another complicating factor. I think you'd only be likely to make headway with consumer rights organizations if there was some real scamming and intentional misleading going on here. This just seems like was just them changing their mind, not a bait and switch thing. I agree that they were a bit sloppy on the timing, though.