I guess some people don't have access to their system prompt, but that's never been the case for me?

I believe at $LASTJOB even Azure's web GUI let you meddle with that. This isn't cast quite so deeply in stone.

That said I do think it's ok that the app developer ask for British English. My overall concern is more that concern about model behavior & expectations feel very tight & narrow across this, and I don't think the typical well defined rigor and expectations if applied will ever ever ever let LLMs on the web. I think sites see that, I think users see that. Making perfect the enemy of the good is going to doom any attempt, forever, and no progress is possible, if these are the demands. We have to be willing to accept that models are limited, especially small local ones. That's ok.