Honorary Knighthood
When a foreign national receives an honorary knighthood of an Order of Chivalry, he is not entitled to prefix 'Sir' before his name, but he may place the appropriate letters after his name, eg Bob Geldof, KBE.
Should he become a naturalised British subject he will be entitled to receive the accolade. Having become a full knight of the appropriate Order, he will then use 'Sir' before his name.