Navigation to the century sub-cats for English, Welsh & Scots will still be easy via Category:British people by century. You claim that this is no longer so. However if you look at Category:British people by century you will see that we have subcats:
style of painting emerged in the British Islesbut then you go on to describe the scope as
the constituent nations of the 18th-century UK. However there was no
18th-century UK; the 18th-century Kingdom of Great Britain did not include Ireland. --BrownHairedGirl (talk) • (contribs) 17:01, 14 May 2018 (UTC)
It's your assertion that these categories are about nationality and not identity.
we don't take a legalistic view of nationality; we just follow what the reliable sources say about nationality. So your denial that this is about nationality is plain nonsense. (If you want to claim that Category:Painters by nationality does not actually carry its plain English meaning of "painters by nationality", then go ahead.)
subset of Europe, without explaining why painters need it, but every other occupation is categorised without such a subset. Bizarre.
where appropriate. And we do indeed know why that doesn't extend to people: we categorise people by nationality, and an archipelago is not a nationality. The rulers of one country in the archipelago have spent over 800 years trying to make the archipelago a nation, and while they came close for 121 years, they never even got as far as even establishing a commonly-accepted demonym. Ninety-five years after the end of that era, it is disappointing to see some editors of a NPOV encyclopedia trying to extend that notion of an archipelagic nationality outside of its actual period of historical existence. --BrownHairedGirl (talk) • (contribs) 15:21, 18 May 2018 (UTC)