[CamouflagedKiwi]

That's not really true. The king conceptually has a lot of power - he appoints the Prime Minister (which can be anyone he wants it to be) and can effectively dissolve Parliament whenever he wants. He is also the head of the armed forces, who all swear allegiance to the King, not to Parliament or whatever.

In practice, this power exists on the understanding that the King won't actually use it, but they are powers that he does have.

[chippiewill]

That's not true these days, the King doesn't actually have those powers themselves - even on paper.

The power actually sits with "The Crown" not the King personally. "The Crown" is a legal entity that is represented by the King but not actually wielded by them. The Prime Minister advises the King on use of those powers which is what actually creates the legal conditions where they're effected by The Crown.

A good example of this is when Boris Johnson unlawfully prorogued parliament in 2019. The Supreme Court ruled that his advice to Queen Elizabeth to prorogue parliament was unlawful, and therefore "The Crown" could not have prorogued parliament and parliament was never prorogued. This would not be the case if it were a power that the Queen exercised themselves.

If the King were to attempt to dissolve parliament without advice from the PM by generating an order in council and sending it to Parliament then the supreme court would simply rule that he hadn't dissolved parliament.

[LeonB]

If the King — without the support of the general populace or of the parliament, but acting in his own interests, like a king of old — dissolved the Parliament, he could and very swiftly would be ousted, legally.

The line of succession would be followed until an individual was found who was willing to support the democracy. This was proven in 1685.

They swear allegiance to the King (or Queen) — but it’s understood that a new King or Queen can be swapped in. The extremely stilted and socially restrained manner in which Queen Elizabeth (for example) behaves is because they entirely know that their family does not hold the nation hostage, it’s quite the opposite.