I’ve long puzzled over this line, & have come to the conclusion that in searching for a flower I was barking up the wrong tree, so to speak. The flowers in the song are mentioned by name, but no blossoming plant corresponds to ‘beauty of Kashmir’. But what does correspond, in both growth & beauty, is a tree, the Kashmir cypress, with its graceful weeping habit. It would certainly be present on English estates by the time this song was written, & would no doubt be much admired.