> I'm sure it's not as believable to an actual British person.

FWIW: As a British person, most of TTS British voices I've tested sound like an American trying to put on something approximating one specific regional accent only to then accidentally drift between the accents of several other regions.

Interesting. While I don't think I could put a finger on Siri's American regional accent, it isn't egregious enough that I ever thought about that.