Can you provide examples of how it curbs extremism? The Tories have been doing extreme stuff and I don't see any intervention from the monarch.
What was intended to happen happened anyway.
The constitutional outrage was failing to implement the 2016 referendum that the people voted for. MPs acted with impunity and were rewarded by ejection at the following election.
The Dissolution and Calling of Parliament Act then set the matter on a statutory basis so the courts cannot interfere in the political process again.
However, even if you are right and it is not extremism. That doesn't support the idea that monarchies prevent extremism. If the argument is, there has never been extremism in the UK because of the monarchy and therefore I cannot give examples of extremism then that's a specious argument.