Personally, I've always thought that she's saying outloud what Marianne has said over and over and isn't saying this time because she so wants to go to London. In a way, she's trying to protect Mrs. Jennings over what she is sure (and is proven to be right on) Marianne's total lack of regard for the woman willing to take them both to London and put them up. I've always seen it as a pre-emptive strike against Marianne's coming rudeness!