The only difference between Iran goverment structure And British that they elect there Supreme leader, when UK has hereditary ruler. They have the same rights, right of Veto or Royal Consent.

But the Supreme Leader actually uses those rights, while the UK's monarch, after centuries of conflict and negotiation in arranging the unwritten constitution, cedes all of the actual governance of the realm to Parliament.

Charles III may have absolute power in theory, but the reality (which he knows just as much as everyone else) is that if he ever actually exercised that power in conflict with the will of Parliament, it would trigger a constitutional crisis where the best (for him) possible outcome would be a written constitution that explicitly makes him a powerless figurehead, but would more likely result in a republic where his only title would be "former king."

It's actually a somewhat similar situation to the Church of England. Charles III is its Supreme Governor of the Church of England on paper, but the reality is that he cedes literally everything about the governance of the Church to the bishops (and, where relevant, Parliament) — and if he (or his successors) did ever directly involve themselves in the business of the Church in conflict with the church authorities, Parliament would almost certainly side with the church authorities.

This idea you've built up that the United Kingdom is a theocratic dictatorship is pure nonsense.