I don't think anyone would know it under its original title!

Anybody who's seen a production of "Oh What a Lovely War", or watched the film would be likely to know it as "On Sunday I walk out with a soldier." And it gets sung often enough on Music Hall nights.

True the Boy Scout line implies after 1907 (though it's easy enough to slip in a line to bring a song up to date), and there's "The Passing Show of 1914", but again that's not conclusive evidence the song was new coined at the time. Likely enough it was though, unless evidence to the contrary turns up.

As fpr that alleged precursor from the Peninsular War which I quoted - I'm a bit sceptical about it, but it'd be interesting to follow it up; with the implication, if it were true, that it'd be the "Sunday" song that was the parody.