George Osborne is the eldest son and heir of Sir Peter Osborne, 17th baronet, and descended from Anglo-Irish aristocracy.

Sir Titus Salt was not knighted, but was a Baronet. A Baronet addressed as "Sir" similar to a knight but they rank above knights. Sir Salt was made a Baronet in 1869.

Not necessarily; you might be a Baronet and have inherited your title. Incidentally, when stating the name of a Knight (or Baronet), place NO emphasis on the 'Sir', which is pronounced 'suh'. Sir Charles, for example, sounds like S'Charles.

Only if the holder is a Baronet.

A knighthood is not a hereditary title, but a Baronetcy is.

A baronet is addressed as "Sir", and his wife as "Lady". Exactly how to address the envelope is a little dicey , but "Sir John Doe and Lady Doe" would be acceptable, I would think.