[onei]

The UK monarch's power is largely based on convention more than active decision making. For example, a government is formed at the invitation of the monarch, but that's long reflected the results of an election. Getting rid of a PM generally happens when they run out of luck. That sometimes coincides with the ruling party/coalition imploding. The next PM is then shortlisted by MPs and selected by a minority of the electorate.

I guess the US equivalent is the leader of the house being unable to hold their majority together. In some ways the presidential election feels more democratic if a relative outsider (like Trump was) can win. But a 2 year lead up is crazy.