I thought the entire point of a jury is that they do live under a rock. You seek out people who know nothing about the case, and then (if necessary) sequester them to ensure that they continue to know nothing.

As I understand it, lawyers seek to ensure that anybody with any domain knowledge is excluded. The jury is intended to be informed solely by what the lawyers present.

I know even less about UK law than I do about American law, so I could easily be wrong here. But from what I have heard, "not your concern" is exactly the design of the system. That system failed in this case, and I imagine they'll be trying to figure out how (and prevent that in the future).