I don’t know much about the monarchy in the UK, but my understanding is that it only exists as a figurehead, basically at the whim of the electorate and Parliament. I don’t think that the Queen could legally “veto” any serious motion (by Parliament) to abolish the throne.
But otherwise, the old advice still holds about intending to kill the monarch and being sure that you actually do that. (I want to state categorically that I am not advocating the assassination of any monarchs or other political figures. It’s tempting sometimes, but I haven’t gotten there yet. And the Q of E seems to be a nice lady.)