I’m seeing a lot of international messages getting this wrong, so this is how you refer to the Prime Minister of the UK.
First, we normally refer to the PM just by name, like anyone else. So, “Keir Starmer” or “Mr Starmer”.
“Prime Minister” is not used as a title like “President” is. He’s not “Prime Minister Starmer”. He’s just “the Prime Minister” or “the Prime Minister, Keir Starmer”.
Unusually, this new PM is also a knight. Of course, this has its own rules.
If you want to use this title, it’s not quite as simple as replacing “Mr” with "Sir’. The first name is more important than the surname here. He’s not “Sir Starmer”. He’s “Sir Keir Starmer” or “Sir Keir”.
Hope it helps!
pedancy talk aside this is like the most inconsistent rule in all of writing hahhah
even the oxford comma is less debated than punctuation in quotes
I just select my precepts from the arguments on offer and try to rigidly stick to them.