A few years ago I looked into how much power she had and I was shocked. I was also shocked by how much that monarchy owns.
And yeah the British monarch's theoretical legal power is immense. For one thing the UK armed forces swear loyalty personally to the monarch, not to the government! The monarch could go to war with Parliament again and the military would be upholding their oath!
The late Queen was a woman of impeccable public ethics though. In some sense it's more admirable for a person who is not bound by the law to choose to follow it scrupulously, which she did.