We recently had a significant test of this. Boris Johnson asked the late Queen to prorogue (shut down) parliament in order to prevent debate on the Brexit negotiations between the UK and the European Union.
In theory he was asking permission from the Queen. But in practice, everyone knew that the Queen was powerless to reject his request. Even for something as plainly anti-democratic.
The Supreme Court eventually ruled that the prorogation was not lawful.
Lots of people were hoping that the Queen would stand up for the people. It was a complicated moment when she didn't!
https://en.wikipedia.org/wiki/2019_United_Kingdom_prorogatio...