I use "low class" in the context of the distinction in the original question. Generally, I would call "ain't" non-standard, in BrE; sometimes (for instance) BrE speakers whose English is otherwise standard will throw in an "ain't" for mildly humorous effect (e.g. "It ain't necessarily so"). The mild humour depends on the non-standard status of "ain't".
(By the way, although my earlier post has the air of a direct contradiction of yours, you weren't there when I began to answer. So no brusqueness intended.) Even if it were the case, there'd be no problem, that's just normal conversation.
All the best,
Thank you for your clarification. And I do agree with you.