Hey @nickgripton !
Thanks for checking in. Premiere Pro is bascially using the OS spellcheck.
As you found, Microsoft is not flagging a lot of words that are not the common spelling in Britain like "realize" because it is not technically incorrect to spell it that way, even in Britain.
Lots of users are frustrated by Microsoft's choice here. The workaround Microsoft is suggesting is to create an exclude list and Premiere Pro will respect that list.
Instructions:
- Navigate to this folder (you may need to unhide hidden folders first): C:\Users\yourusername\AppData\Roaming\Microsoft\Spelling
- Open the appropriate folder. For your case, "en-GB"

- Open this file in a text editor: default.exc

- Add all your words you want to be flagged as misspelled. One word per line, as such:
- Reopen Premiere and see if you now get these words flagged as misspelled. That's it!
Thanks to Ai this shouldn't be too tedious. I have created a list for you that you could use as a starting point:
https://drive.google.com/file/d/1VtXRQlKZyEeh3wkuqM9C_XN5DOA1rXXX/view?usp=drive_link