I know, it’s why I said a link to one.
I think you’re confused as to what the en_GB version of WordPress should be doing. Here’s an example of what it does do. If the word color appears in WordPress (the actual UI created by WordPress), the en_GB version will display colour. That’s it. The maintainers of the en_GB version have gone through the code and where a word or phrase is available for translation, that’s what they’ve done.
What I think is happening with you is this. You are typing in the post editor and you type for instance colour and you are seeing a word underlined as misspelled. That’s not WordPress doing that, it’s your browser settings, probably set to US English.
I use Google Chrome most of the time. I set its spelling preferences to British English. You can read about that here:
https://support.google.com/chrome/answer/95604?hl=en-GB
If you don’t use Chrome, there’s no doubt a similar page for your browser.