A subject of Great Britain is only entitled to be addressed as "Sir" if he holds a Royal Commission or Royal Warrant in the Armed Forces or Civilian Services. Other Ranks, of which I was one when I served, are not entitled to be addressed as "Sir" unless decorated by Her Majesty.
Civilians engaged in commerce certainly do not warrant the term "sir".
Sorry, but this wrong.
A knight of the realm is addressed as 'sir' on an honorific basis (quoting either their given name and surname or given name only), whereas the term 'sir' is used as a courtesy title to avoid addressing someone by their name or surname.
Military courtesy is another matter, although a military knight is addressed by his professional title before 'sir.'