Okay, the book is written by an English person, right? So it makes sense to me to leave the spelling, punctuation, grammar, and word choice consistent with English usage.
But wait! What if one of the stories is told from the POV of an American? I realize that, if the POV character were French, the story wouldn’t be written in French, but shouldn’t a non-British English speaker’s POV be presented in non-British English? Yes or no? What if the POV is first-person (I liked the colour or I liked the color)?
What if a non-British English character reads something written by a non-British English speaker? Should that writing be quoted in non-British English? What if the character reads it aloud, or what if a British English speaker reads aloud something written in non-British English? Does the writer use British or non-British English for that?
Editing. It’s not all glitz and glamor (or is it glamour?), kids.
A WRITING PROMPT BASED ON MY POST: Write about people who speak different forms of the same language.