This page mentions a WW1-era song called (or perhaps just containing the phrase) "It wasn't the Yanks that won the war!"

There is a tune called "My Boy Willie" performed by (guess who) The Royal Tank Regiment Cambrai Band on their album "Challenge" (2002). It's also performed by the Regimental Band of the Coldstream Guards on "Regimental Marches of British Army, Vol. 1" (1991); and it's on "The Passing of the Regiments" by "various artists."

I don't suppose any of those recordings will have words, though.

I also found a song called "My Boy Willie"

Oh, where have you been all the day, my boy Willie?
Oh, where have you been all the day? Willie, won't you tell me now?
I've been all the day a-courting of a lady gay,
But she is too young to be taken from her mammy.

Oh, can she brew and can she bake, my boy Willie?
Oh, can she brew and can she bake? Willie, won't you tell me now?
She can brew and she can bake and she can make a wedding cake
But she is too young to be taken from her mammy.

Oh, can she mix and can she spin?...etc.

It is sung in an art-song manner by Kathleen Ferrier on "Edition V8 / Blow The Wind Southerly." I'm pretty sure it's the same tune played by the regimental bands listed above, so I'm not sure we're talking about the same song that was requested...

I'd be interested in knowing if the above song is sung to the same tune that your dad remembers. I'm wondering if someone took the tune of the old traditional song and wrote new words to it.