Well, the poem is spoken by Miss Havisham and is about Miss Havisham, so the title seems pretty cut-and-dried to us.
But when you consider the fact that this character is always referred to as Miss Havisham in the novel Great Expectations, the title takes on an interesting new twist. Why leave out the Miss? The missing "Miss" has at least two intriguing effects. First, it takes Miss Havisham's gender out of the picture. When we read the title, we can't be sure this is the Havisham we're familiar with.
Second, by leaving it out, Duffy is, in a weird way, drawing attention to the fact that Havisham is her maiden name. She hasn't taken on her husband's name because she never actually married the guy. So here she is, an old spinster, stuck with the name she's had since birth. It's a constant reminder of her sad, sad life.
And finally, it makes her seem big and important. It sort of puts her on par with characters like Hamlet (whose play, after all, isn't called Prince Hamlet), or Othello (whose play is called Othello, not General Othello). She's a secondary character in Great Expectations, but in "Havisham," Miss H takes center stage.