Well, it falls under the Human Rights Act, so missgenderring would have to be argued to be part of discrimination based on 'gender identity or expression' to be handled through a human rights tribunal, which is generally limited to financial penalties (though refusing to pay could result in criminal court involvement). The good news is that the human rights tribunals no longer have a 100% conviction rate (too much publicity over certain cases in the early 2000s), and overreaches (like ordering an accused person's home seized and sold to pay the complainant) can get overturned by actual courts.
I'm not really seeing why not misgendering someone is considered a problem, though; I mean it's the same sort of thing as using e.g. racist language to address someone. I think the problem comes down to seeing it as being a black-or-white issue, no pun intended, where it really has to be based on all sorts of things including terminology, intent, circumstance, context and so on. While I agree it "should" be the case that it says much more about somebody who chooses to be crass and unpleasant the reality is that it can and does cause actual harm, at least eventually. Making a gamble about when is the cut-off point for "eventually"... I guess it's easier to just be considerate in the first place.
