"The Girl I left behind me" is still a puzzle. Wm. Chappell's PMOT has many notes on song and tune, but nobody has ever been able to verify them (he avoids giving precise sources or dates) or show that song or tune were known in the 18th century. I finally found a tune direction 'To the tune of "The Girl I left behind me"' in a songbook of 1799, and I have a text of the song, c 1806, which seems to the earliest yet found. There's no Brighton Camp in this version.