For the UK, you also need to represent the Celtic languages. You'll need at least these letters: â, ê, î, ô, û, ŵ, ŷ, à, è, ì, ò, ù, ẁ (maybe ỳ?), á, é, í, ó, ú, ẃ (maybe ý?), ï (maybe more ¨), ...

Example: https://www.llyw.cymru/ (Welsh government homepage).

Bits of https://www.highland.gov.uk/press (Scottish Highlands council).

(I'll admit most people who don't speak these languages won't bother with the diacritics, unless it's as easy as typing English.)