"The pipes playing the Scottish lament Floo'ers o' the Forest is redolent of Scottish or expat Scottish troops."

I think you are right and it could possibly be localised even further. Bogle is a Scottish Borderer. The song Flowers Of The Forest originally pertained to the young men of Ettrick Forest who never came back from the Battle of Flodden. The song has a special place in the minds of many people from the central Borders. I did read that Hamish Imlach said that whilst they were touring together Bogle wrote most of the song in a hotel room in Germany sometime after Bogle had visited cemeteries in France. Seemingly the name "McBride" had stuck in Bogle's mind - let's face it probably because of no other reason other than it rhymes and scans with graveside! I suspect that the rest of the detail is all probably poetic license. The song is about all the war dead not just one individual. Whether the McBride in question was actually a Willie - or a Tom, Dick or Harry, and whether he was Scottish or Irish or Martian isn't really the point.