I've done some more testing. I am now convinced changing the default proofing language in Microsoft Office products is a useless feature, with no real effect. Selected Windows input language determines the default proofing language in all new documents.
I also found some Microsoft articles supporting this. For instance:
To change the default language in Office programs, you must change the default input language for the Windows operating system, and then change the default editing language for Office
(Source:
Change the default language for Office programs - Word - Office.com)
But if there is a way of retaining the keys and changing the input language - let me know.... I am still open to suggestions, but as I said I cannot change my US-styled keyboard.
Of course you can. You just don't want to. A big difference there, something being impossible or the user being stubborn.
Although already said but let's repeat:
Your original question: How to change default proofing language in Microsoft Word?
Answer: Microsoft Word uses selected Windows input language as default proofing language. In order to set default proofing language to UK English you need to use UK English as Windows input language.
I guess you did not read the tutorials I linked in one of my
previous posts?
Note
Pleaser read the below very carefully, ask if there's something you do not understand. OK?
Input language and
keyboard layout are
not synonyms, they mean something totally different. An input language can have and use several different keyboard layouts. An input language and a keyboard layout can both be set independently.
See below, remember that as my
Taskbar is on the left your actual location of the language selectors is bottom right instead of bottom left as in these screenshots.
If you have installed two or more keyboard layouts to an input language, the language selector has two parts. A two letter abbreviation for selected input language, and a keyboard icon for layout:

In my case I want to have US English as input language by default but use Finnish keyboard layout. To achieve this I select first the input language:

... and then the keyboard layout within that input language:

As you can see from the screenshots, the empty word document next to the pictured Taskbar notification area shows that Word uses US English as proofing language although the keyboard is set to Finnish. This is because the input language determines the proofing language, not the keyboard layout.
The same might be easier to understand if you look at the Language Bar instead of notification icons:

Select input language on left, keyboard on right.
To achieve this, setting the default input language to US English and default keyboard layout to Finnish I have simply installed needed input languages and keyboard layouts, then selected the correct combination in
Regional Settings:

Now in this my case all new word documents use default US English as proofing language even though I am by default using the Finnish keyboard layout.
In your case, setting the defaults to UK English input language with US English keyboard would do exactly what you wanted and came here to ask.
Kari