I have often thought that some of the London crowd, the younger generation in particular are exactly as you say when hanging out together, but I don't think it is because they don't know any better. The Bingley sisters went to one of the "first private seminaries in town: where they would have been given a good foundation of how to represent themselves well in society. Darcey was raised by a well-heeled, prominent Derbyshire family who I am sure did not neglect to give their son the rules of good behavior as well as sending him to a good school which would reinforce this.

Growing up, our parents taught my sisters, brother and myself how to speak and conduct ourselves properly but when out of earshot/on our own we indulged in slang and slouched into teenage expressions and such just like any other young person. Even today, when talking to each other we are inclined to talk in slang the same way we did as kids but when we get into professional/formal/unknown territory we change gears to suit the environment.