I didn't say it wasn't a British stereotype, only that that particular British stereotype is not one that says "innit".

There's a stereotype of stereotypes that there are only two English accents: "aristocratic" and "cockney". The film Mary Poppins may have a lot more to do with this than it ought to.

Anyway, the Countryball's appearance is associated with the aristocratic. "Innit" is cockney. (This is overly general to the point of being wrong, but for the sake of this argument, it's close enough.)