The monarch does retain some theoretical power. Only she can appoint the PM etc. Though in practice there are precedents laid down saying who is appointed etc and she follows these to the line. So she basically acts as a rubber stamp. It is the duty of Parliament to act in a way that never puts the monarch in a position to use the power but it is the monarch's duty so to speak to ensure that there is a gvt should there be a parliamentary impasse. This has never happened as yet and everyone knows that actually using the power would cause a constitutional crisis. So it works as long as it works.