Peoples lives experiences are different.
I play a multiple choice trivial pursuit based game on my phone, I'm good on history, entertainment, arts and not awful on the science stuff,struggle a bit with the geography (a common "question" is you are presented with a cityscape, either one photo or a montage - like those postcards you get showing the city's main landmarks) and I don't recognise the less well known ones eg and I'm hopeless on the sport questions!
Nobody knows everything and being smug about your own knowledge is actually quite arrogant!
"But we all spend more than a decade in full-time education" no there are plenty of people who don't attend school and aren't educated at home.
Chernobyl was meaningful to people of my generation and older, but I wouldn't necessarily expect under 35's to know much about it at all. Because the disaster didn't happen in their living memory time span.
There are events that happened which for my parents generation were huge but which my generation and younger know little if anything about.
Wauden - what do you think a costume drama is?!
"As a Scot, you can't imagine the number of English people who have no idea about how the UK is formed, about the Troubles in NI, that we don't have separate passports, that we used to be separate countries or that we aren't actually ruled by England. I mean, it's literally like 90% of English people I meet." I get what you mean but as a Scot educated mainly in England I understand why. The history and geography I was taught there was very England centric with huge gaps and inaccuracies.
I too was raised if you don't know something go find out. Parents both left school at 15 but well read and politically engaged. We were encouraged to ask, look up in pears or other books at home or look up on next trip to library, if we asked them something and they didn't know they didn't make a big deal of it just "you know I'm not sure, I'll try and find out" sometimes this involved asking their own friends and family because they knew where their knowledge generally lay.