When you have a monarch up top, there's no question that a PM can be held to account. If the PM is corrupt, they are out.
The monarch provides the source of trust from which democracy, the government, law and all power flows, and yet they exercise none of it.
It makes situations like the one with Trump simple to treat. We distinguish sovereignty and the operation of government. It's a harder question to deal with in the American system which combines the two roles.