Incorrect. The monarch still holds executive power within Britain; it is not ceded whatsoever. Withholding royal assent is the equivalent of a veto. The parliament cannot stop the monarch from doing that, and the monarch is absolutely not duty-bound to rubber-stamp any and all legislation that comes out of the parliament.
She is, if it's constitutional. She has reserve powers to block unconstitutional laws, but no others.
She also, expressly, took an Oath at her Coronation that cedes any power she might have to intervene on religious or moral grounds to the Bishops and Archbishops in the House of Lords.
The Queen is not a tyrant. It amazes me to see the number of American Republicans at the moment who seem to think she should try and act like one and overrule the will of the elected Parliament of the United Kingdom. Didn't your forefathers fight a war because of something like that? Isn't that why America has Republicans?