@Marc NobbsHere's an interesting article on differences between US and UK spelling from the perspective of publishing.
https://www.lisapoisso.com/2017/12/06/american-vs-british-english/
Here are some snippets from the article:
But in publishing, conventions of usage characteristic of regional language (whether that's American English, British English, Australian English, Canadian English β¦) are handled with more care. Amazon has been known to warn authors and even remove books from sale due to "errors" in books in cases of mixed American vs. British English. It's an important detail for professionally published books.
We don't want your readers calling the whole thing off either, but that's what can happen when a writer pulls the audience out of a story by peppering the sentences with unexpected inconsistencies.
And that's where spelling variations can be problematic.
Here's the thing. Your preferences and my nuances mean nothing. When it comes to novel craft, it's all about the reader.
I'm not saying readers aren't flexible. If a novel's written in British English, fine. American English, no problem. But a mixture of the two? The reader who doesn't know the variants or industry-recognized standards won't notice and won't care. They'll bend any which way.
The problem is the readers who do know. They won't see your preferences; they'll see mistakes. They'll think you didn't hire an editor, or you did but you didn't take the editor's advice, or you did but the editor was rubbish.
Ask yourself thisβdo you want your readers thinking about the degree to which you invested in quality control for your novel, or do you want them immersing themselves in the world you've built and the characters who move around in it?
If you're serious about building a loyal audience β¦ Be consistent and respect convention. Spell in a way that'll keep their attention on your story.
Now since this is the 6th book in a series and the others use British spelling, why would you change that now? Just use British spelling throughout like the other books.