You relate your own experience in UK. I can relate my experience in France. Background checks of criminal past are extremely unusual. What is usual is to contact former employers (which is something I did when we hired Sue and Mike for example).
In my experience, it's the same in the UK. I think people are failing to consider the fact that while Wikipedia is enormous, the WMF is tiny. Yes, she had the title "COO", but that's because she was the only person in her department, so she had to be in charge of it. There is a big difference between the COO of a large multinational corporation and a solitary bookkeeper of a one-office charity.
That said, it is common practice in the UK to *ask* about criminal background. Did anyone do that? If she outright lied to you, then it's not really your fault. If you didn't think to ask, that's negligent.