A minor conjecture, but no one seems to have thought of it before:

When Fred Godfrey wrote in 1941 that the he had written "Bless 'em All" in WW1, he added, "And, furthermore, it wasn't 'Bless.'"

Since Godfrey says his (semi-official) RNAS vaudeville troupe performed the song for "hundreds of lads every evening," is it likely that their superiors would have allowed them to sing, night after night, anything stronger than "Damn" or "Blast"? In 1917-18 it was still illegal in Britain even to print the familiar "F-word."

Which would have been added in the "folk" versions, sung in uncensored situations.

I wonder if Godfrey's sense of propriety would have permitted him even to have alluded to the F-word in a letter to a general circulation newspaper in 1941 - particularly if he was suggesting he'd sung it out loud, on a stage, many times!

(Note the resemblance between "blast" and "bless," which may have replaced it.)