It’s an interesting question, when looked at more broadly. Should it really matter if US spellings are adopted widely in the UK?
After all, language evolves and it’s fine, really, that certain expressions that were once seen as very American have been adopted here, for example “regular” meaning “standard size/medium” or normal. Or “allowance” instead of pocket money. We’ve been exposed to US culture for decades via films (which we now mostly call “movies”!) and TV (think how we talk now about “seasons” rather than “series”.)
So how does US spelling fit into that? I think that young people are seeing a lot more written US material than they used to, because of captions on TikToks etc, or links to US websites.
Should teachers teach that both “defence” and “defense” or “color” and “colour” are acceptable in writing produced by a British person, as long as the writer picks one and uses it consistently? I work in law and would find it very odd indeed if an English judge suddenly started writing “defense” in a judgment. But we already have it with things like “s” and “z” being seen and more or less interchangeable (“organisation” vs “organization”). Some of the American spellings are, of course, just antiquated British English ones anyway and I think we accept z in that context already because it used to be used here.
I am not anti-American and I can’t quite work out where my deep-rooted objection to our adopting their spellings comes from. I think it is a case of us having rules and it being annoying that people are either forgetting them or deciding they don’t matter.But we don’t have an Académie that governs language like the French do, and the approach of the OED is to document language as it evolves, not prescribe it, so is there really such a thing as a spelling rule at all?
How do you even define whether writing should be British or American? Is it the nationality or residence of the author or the nationality of the audience? Audiences are indeed mostly global these days. What if an American working in a British university writes a report for the UK government? Will Nat West just say that their app was developed by American developers?