The RAF Warrant Officer - according to the RAF website
Warrant Officers (WOs) are often included in the SNCO category, but actually form a separate class of their own. A Warrant Officer will have many years experience and is respected by both rank structures. Warrant Officers are addressed as "Mister" (or "Mrs", "Ms" or "Miss" for female Warrant Officers) by commissioned officers (and as "Sir" or "Ma'am" by everyone else). SNCOs and WOs have their own messes, whereas JNCOs live and eat with the junior ranks.