How can I spell-check in both en-GB and en-US?

I am a real newbie, both to Linux (Mint, Cinnamon) and Libre.
I am in the process of writing a book which contains words that I know will not appear in any UK or US English dictionary, plus, of course, typos made during the normal task of writing a book!
When I click on the spell-checker button, I’m politely asked if it should continue checking from the top of the document, and when encouraged to do so ALWAYS comes back, proudly stating that there are no errors. Well … I KNOW there are, but it’s not finding them.
What am I doing wrong?
I have selected UK English as my language, but have selected US English as the text-entry language as it’s the only one that had a spell icon next to it.

Check-list of required options in this answer. Note that Ubuntu / Mint provide their own variant of LO that can have problems quite separate from the website version.

The dictionary you need must match the language you are writing in. During the installation process you can select all kinds of dictionary.

Fro additional dictionaires, have a look here:
and here: Selecting the Document Language - LibreOffice Help

I am using an US English interface and write in English, German, Dutch with dictionaries for each language. Only Japanese doesn’t have a spelling dictionary as it is not possible to use one.

let me add a comment: For writing a book I recommend to learn to work with a master file:

You can download the all guidlines here: