It isn't intended to be a folk song. It's Grainger's most complex composition, with multiple layers of meaning, including some very dark autobiographical associations.

Art music (or jazz, or techno, or...) which uses folk themes does not thereby become folk music. (As decided by the Church in the arly 15th century, when they agreed that settings of the Mass using popular songs like L'Homme Armé were legitimate liturgical music).