US Presidents are more-or-less above the law, as interpreted by the Justice Department. That means there really isn't a full-on rule-of-law thingy going on in the US. Trump organised a White House party while he had covid and gave it to numerous attendees. No consequences.
In Britain, the PM is subject to law and may lose his job because, for 25 minutes, he attended a socially-distanced garden party organised by someone else. Other attendees may be fined.
Why? Because the queen is the head of state, and all the deference goes to her. She is the source of trust in the system. The PM is just an ordinary person. The rules apply to him.
If we had a bad monarch, things might be different. But the queen is very good at what she does. Hope Charles, William and George will be also.