The British monarchy is unlike most other monarchies in that the person of the monarch is simply the symbolic 'hand' of the institution of the Crown. They have no power of their own. None.

In the British system, the Crown is the source of all authority, but that authority is not exercised by the monarch, but by the Courts, the police, the Civil Service and Parliament. The monarch cannot make or veto laws, nor can he or she enforce them. All the monarch does is sign a Bill to make it an Act, make the Speech laying out the legislative programme decided by the government, and ask the leader of the majority party in Parliament to form a government after an election. All things doen on behalf of the Crown, and over which the actual person of the monarch has no say or control.

It is this structure that stops Boris Johnson going Trump on us. There is only so much he can do, and he has to answer to Parliament for any and all of it, if asked. British MPs are notoriously less likely to toe the party line than American Representatives or Senators, so Boris, or any British Prime Minister, is always one vote of no confidence away from replacement. No lengthy and risky process of impeachment necessary.

So while the hereditary monarcy has been a tradition since 1066, it only remains so by means of certain Acts of Parliament, any and all of which could be repealed and replaced by, say, an elective monarchy, at any time. Should the Windsors become too much trouble, they can be replaced without materially changing the way the government works.