Constitutional monarchies, such as the ones in Europe have no political power, but they hold back certain powers that an autocrat might try to exercise. If a truly autocratic leader tried to take control in Britain, the parliamentary system would be the first hurdle (but we see how the emasculated Congress and Senate have failed in the US), but they would also need to remove what is still a popular institution in the monarchy. I am a monarchist, not a royalist and take no part in the regular obsequies that the country is prone to, and would prefer a non-political Head of State.
