in the USA..
Recall seeing that movie documentary that was out this past year, that was colorized videos from WW One..
the biggest thing that stuck out at the viewer, was in close up shots of men's faces, immediately you could see the bad teeth just about everyone had... and most of these pics were soldiers from then late teens to early 30s...
a mouth full of all their teeth and in good shape was a real rarity...which I gather wasn't much different in this country in those days...
The movie is "They Shall Not Grow Old", and let me tell you I certainly took note of the kids in their late teen with completely destroyed mouths from caries!
There are a number of factors that feed into the classic "Brit Mouth" syndrome. In the first half of the 20th Century, dental care was pure episodic. Poor went only when something hurt. The solution was extraction. That leads to problems with kids. The concept of just pulling the primary teeth because "they will lose them anyway" leads to space problems. Primary teeth need to be space maintainers to keep the erupting permanent teeth from erupting into the wrong position. If there are no primary molars, the first permanent molars erupt in, migrate forward and then horrible crowding is the result. Take a look at early photos of the Beatles and Stones. The crowded and misaligned teeth were a result of them having their primary molars pulled when they were young.
No fluoride. Britain was slow to adopt fluoride in municipal drinking water. I had a professor, Dr. Victor Della Gustina that worked with the NHS in Scotland to get that going for them in the 1970. Nice likable guy, he had some hilarious stories about the Scots resisting fluoride, thinking it would make their whisky and water taste funny. Anyway, kids who have fluoridated water have hard enamel and less caries by far.
Then their socialized medicine, the NHS, is just basic bare bones care. People don't make appointments, they just show up and wait to be called. The British approach to dentistry is still largely episodic, although the younger generation is embracing prevention.
Lastly, the British Isles have a mix of Celtic, Viking, Saxon, roman and who knows what other DNA that seems to give them a mish mash of mismatched genetics for upper and lower jaw size. More so than I see in French, Germany or Scandinavians as a rule. Africans rarely have that crowning problem, for example,e. Nice wide dental arch and big thick enamel and long roots. Asian have thin enamel and wimpy roots. There you go!