But if someone was going to do something wholly unconstitutional how would a head of state stop him? What would the process be
How would the Monarch stop them?
A Head of State and officers in the Armed forces swear to uphold the Constitution.
This is what the Queen swore
Archbishop. Will you solemnly promise and swear to govern the Peoples of the United Kingdom of Great Britain and Northern Ireland, Canada, Australia, New Zealand, the Union of South Africa, Pakistan, and Ceylon, and of your Possessions and the other Territories to any of them belonging or pertaining, according to their respective laws and customs?
Queen. I solemnly promise so to do.
Archbishop. Will you to your power cause Law and Justice, in Mercy, to be executed in all your judgements?
Queen. I will.
Archbishop. Will you to the utmost of your power maintain the Laws of God and the true profession of the Gospel? Will you to the utmost of your power maintain in the United Kingdom the Protestant Reformed Religion established by law? Will you maintain and preserve inviolably the settlement of the Church of England, and the doctrine, worship, discipline, and government thereof, as by law established in England? And will you preserve unto the Bishops and Clergy of England, and to the Churches there committed to their charge, all such rights and privileges, as by law do or shall appertain to them or any of them?
Meanwhile the Armed Forces in the UK swear
I, (Insert full name), do swear that I will be faithful and bear true allegiance to Her Majesty Queen Elizabeth II, her heirs and successors, according to law. So help me God.
Meanwhile, in a country with a different system:
This is what the Irish President swears
"In the presence of Almighty God I do solemnly and sincerely promise and declare that I will maintain the Constitution of Ireland and uphold its laws, that I will fulfil my duties faithfully and conscientiously in accordance with the Constitution and the law, and that I will dedicate my abilities to the service and welfare of the people of Ireland. May God direct and sustain me".
And the officers in the Irish military swear
do solemnly swear (or declare) that I will be faithful to Ireland and loyal to the Constitution and that while I am an officer in Oglaigh na h-Eireann I will obey all orders issued to me by my superior officers according to law and I will not join or be a member of or subscribe to any organisation without due permission.
Personally, I prefer the idea of upholding the Constitution than being loyal to a Monarch