They... didn't get a refund, though. They got a £20 account credit (which is not a refund), and still no access to at least one of the films.

This is why we pirate movies (and books, which have the same problem on proprietary e-readers). It's not about getting things for free, it's about actually owning the thing we bought.

In similar situations how hard is it to initiate a class-action?

only coz he made a loud mess. Imagine tens of thousands or more that can just get bent.