You forgot to add none other than himself "Rudy Guiliani to that list. After 9-11, while still in office he travelled to England, with the NYC Fire Commissioner, and was dubbed a Knight.

George Bush waited until he was out of office. If he was still in office it would have been an offense. Rudy Guiliani was still in office, but he was not holding a Federal position/title.

Correct me if I'm wrong, but this only applies when you are holding a Federally elected position and Congress can make an exception for you beforehand, according to the US Constitution.