So are all metaphysical belief systems, henry--even your socialist views have a root in a commitment that you think you're right. You make a commitment to a belief. Just as every religion in the world makes that same kind of commitment.
So in legal jargon, she holds back because every monarch since Queen Anne is afraid of the block. If it's not on paper, it's not official. It's just been "convenient" for all parties. As however the Supreme Commander of your military, do not tell me she could not utilize the military as she sees fit. She only leaves it all to Parliament because A) she was raised to go along with Parliament and B) She hasn't the need for power as her predecessors did. I'd say out of the past 5 monarchs, Elizabeth has been the most "restrained" monarch. I doubt you'll see that same level of restraint with Charles who WILL want a more active role in the government, and without a piece of paper codifying his exact powers, there isn't much you can do aside from Civil War to stop it. I can almost guarantee her father would never have granted royal assent to joining the European Union, however much he hated being king.