Daphne's Title...
... AKA I'm Thinking Way Too Hard About This
As Henry is Lord Dashwood, Earl of Wycombe, Daphne is entitled to use "The Lady" before her name: The Lady Daphne Reynolds Dashwood. Even though her parents divorced/split, as long as she was a legitimate child (and the movie implies she is), she would get to use the title. This should have been used at the very least on Daphne's debutante ball invitation.
Of course, Daphne wouldn't have known about it before arriving in England, but one would think that Jocelyn would have mentioned it. (Goodness knows Glynis or Clarissa wouldn't have.)
For more information, look up "Earl" on Wikipedia, and picture Colin Firth in that sweet coronation robe and coronet.