Thank you so much for this! Definitely seems likely I just missed out on the joke.

I would guess that this is sort of an embedded joke about the use of British regional language/accents as a world building framework? A brief google suggests it is not from the books.

I am of the age/schooling/class background where people make this reflexive correction entirely unironically. I know several older people who do this, and I remember the cultural campaigns over the “ten items or less” signs in supermarkets.