—“What legal grounds can the legitimacy of the extant commonwealth sovereign (the Queen of England) be revoked?”— A Friend

There are only three conditions:
1 – If you hold a constitution of natural law (like the USA, and less so the UK), then for the systemic violation of that law (this is the virtue of absolute constitutions).
2 – if you do not hold a constitution of natural, then for the systemic violation of that constitution. (This is the problem with populist constitutions).
3 – If the sovereign attempts to alter the constitution without a substantive (natural law/common law juridical), or legislative procedural (continental), or populist (democratic approval) justification.

And the three criteria are: Treason(conspiracy), Usurpation, Circumvention. Ill judgement is not a criteria. Disagreement is not a criteria.

The purpose of the monarchy remains, as does do lords in the UK, Senate under the old US constitution, Judiciary in the current US constitution, defenses against the ‘populism’ of the people. This is the best defenese against the ‘passions’ of the people. (ignorance and folly)

The second best defense against misrule by the people is the demand for reversibility and restitution for bad policy, legislation, and law. This has not been yet implemented in a constitution that I know of but it would end most nonsense debates by warranty (“skin in the game”).

I have never seen another reason to revoke the legitimacy of a sovereign, only to replace the sovereign. The process of replacing a sovereign is quite simple and common: Regicide. Regicide is most often performed by members of the royal family, out of familial defense from the public anger at a monarch.

Let us recall that anglos have the longest continuous governments extant for the very simple reason that our governments from time immemorial out of necessity of dependence upon the militia for defense (and aggression), is contractualism. And that while we have had many civil wars in our history on both sides of the atlantic, the only substantive change to prevent them and to end them has required modification of the written contract that limits the powers of the government over the militia (citizenry).

(Populist Brits are insane. Monarchies are priceless assets.)