Just heard this for the first time on Penguin Eggs - where have I been all these years?
So, my first impression is simply this . He goes to his love, to tell her he is gong away ( and by the way I am sure the words are to tell her "I'm to" Flandyke shore - in other words "I am going to" - and then "unto " Flandyke Shore) and never coming back ( I assume he is going to the Low Countries to fight). She is broken hearted, Next time he goes he is actually seeing her ghost because she has killed herself out of grief for his leaving. Hence the sun-like aura.

Her father , himself stricken with grief at the death of his daughter as a result of his having left her, never to return, seeks him out to rage at him or maybe share his grief with a man who had loved her.

The pathos of the last lines is the sheer helplessness he feels at the girl's death that all he can do is make a futile gesture and shoot a bullet back to England - perhaps as a last farewell to England and to her. For some reason, the futility and throw-away nature of this last gesture I find quite moving.