The White House Press Office announced in their writeup that Prime Minister Teresa May (British porn star) was coming NOT Theresa May (actual Prime Minister). Could that be why the press conference lasted 18 minutes because Trump read what the Press Office wrote and expected Teresa May, thinking “oh porn actress became Prime Minister where are the breathe mints” He did looked annoyed at the news conference and this would explain it.
If this was the Bush W Whitehouse I would excuse it as a stupid mistake. Trump White House? Not so sure.