How do I change the default language

Tools | Options |Language Settings | Languages | Default Languages for Documents
then set Western to English (UK) would seem to be the route. Then Apply and OK and then Save and exit document.
However, when I open a new document the language is still English (USA)

Tools | language | For All Text - offers English (USA) and English (UK). I can switch to English (UK) for an individual document, either by going through the rigmarole above or by right-clicking English (USA) at the bottom and selecting English (UK). But I still can’t get English (UK) as default. Since I have work-arounds it is but an irritation.

OS: Currently Windows 10 Pro 20H2 - recent (this week) up-|-down-|re-grade. Formerly W7 Pro. LO: (64)

This is the correct way. But, though it is not necessary, have you installed the en_UK language pack? It may depend on OS (which you didn’t mention nor your LO version). Instructions for installing the English variants vary with OS.

In addition to @ajlittoz,

Some time ago I had a similar problem with changing the default font. Following the Tools|Options route did not work. Eventually I was given advice on how to do this. I had forgotten until a few hours ago. I found my notes and the method, curiously complicated, works (at least for me).

Change default language and font

  • Create an empty text file on the desktop with the preferred language setting and save.
  • Goto C:\Program Files\LibreOffice\share\template\shellnew
  • Rename the file soffice.odt to soffice.odt.old
  • Change the name of the desktop file to soffice.odt.
  • Copy that file to C:\Program Files\LibreOffice\share\template\shellnew
    Note: you will need administrator status to do this.

I bet you create new documents direct from the desktop without opening first LO. There is a known bug in Windows where this direct creation will not invoke custom template but only the default template. The bug is not on LO side but on Windows side. This is why the complicated workaround is needed.

Since it is a template, I wonder if the extension should not be .ott (instead of .odt).

Thank you. That probably explains it. I almost invariably create new documents with Right Click and New. The workaround obviously edits the default template.