By Jennifer Newton Prince Harry once had a surprise response when a close aide accidentally called him “mate”, according to a royal book. Before his marriage to Meghan Markle in 2018, Harry shared many of the same staff as his older brother Prince William, when they were both based in Kensington Palace. Their office was seen as more informal than that of their father King Charles, who was still Prince of Wales at the time, where formal greetings such as “Your Royal Highness” (before he ascended to the throne) and “Sir” would be used. One former press secretary at Kensington Palace, Nick Loughr…