King Charles uses a variety called Received Pronunciation, but both of his sons (William and Harry) use Southern Standard British instead. Geoff Lindsey has a video on the differences.
As such, once William rises to the throne, what's considered "the King's English" will change. And, alongside it, what plenty people in the UK consider as standard English will change too.
Slang is slang. It's always used verbally. I am not sure why someone would expect a llm to generate proper slang. Not sure at all how stating that fact makes one a "bigot".