[matthewheath]

It's one of her reserve powers and exercise of her powers is non-judiciable (cannot be reviewed by a court); only the advice given to her can.

If she so wished, given she is the font of all law in the country, she could close down Parliament and sack her advisors - there is no legal mechanism to stop her, except the practical consequences of doing so.

[underwater]

You could argue that there is a difference between practical and theoretical power, but all laws are imaginary and exist only in our minds.