It is Marks & Spencers responsibility, even if there's nothing they could have done differently. You're their customer. In this scenario, when the DVD they sold you didn't work the way you wanted it to, they are the ones you should complain to, and they should give you a refund.

Yes, it's their responsibility. And that's why they put a media region lock sticker on the box of the DVD. (And usually it's also printed on the labeled side of the disk.)

Legally, Marks & Spencer must put this restriction into their contract with the customer. And of course these are sometimes implied contracts. Very much based on jurisdiction, case law, etc.

I'm not talking about law, I'm talking about what customers should expect from merchants. Even if there's some fine print somewhere that says otherwise, I expect the thing I bought from a merchant to work for me, and if it doesn't, I expect my money back. If neither of these things happens, I'll no longer be a customer of that merchant.