The king has veto power over any acts he believes affects him, and him and the queen have apparently used the threat of this many times in order to shape UK legislation before it's argued on. Apparently almost all legislation is passed by the royal family's office for edits before it goes up for debate.
https://www.theguardian.com/uk-news/2021/feb/08/royals-vette...
This idea that the royal family has no meaningful political power is not congruent with how the British political system works.