If a branch is considered incorrect, then all languages except the first individually developed languages are also incorrect. This means British English is incorrect, and also all English. German is too, and French is too. Swedish, Dutch, Vietnamese, and whole other slews of languages are incorrect. Sorry to be a party pooper.
English has roots in Latin, French and Germanic languages - and they all have roots in proto-languages. I understand it was cheeky rather than dismissive, but the Brits often confuse this.
The truth is that all languages borrow, bastardize and butcher other languages. Just look at the VN word for television - tivi; the word could be reflective of what it does (hộp ảnh?), but they borrowed from American slang (British would be 'telly').
My point is that we'll never anticipate where our languages will merge, and once they diverge we can just consider the new version as correct as the old version. With this in mind, all slang can be considered correct as well. Works for me.