RE: It is important to note that "The Ashoka" is (or was) a very famous Indian curry house in Glasgow. My own experience of this dates back to the years 1986/7. It was well known that it was the best curry house in Glasgow (and therefore the best curry house in the world).
It seems that this constitutes incontrovertable proof that the tune is indeed based on a Scottish air. It is of little surprise that the "Farewell to the Ashoka" (or "The Ashokan Farewell") is a lament.
That restaurant, according to its website, opened in 1984. The song was written in 1982.