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I believe the rank of guardsman was given by King George V just after WWI, to honor the sacrifices of the Foot Guards regiments during the war.
It certainly was in use prior to WWII.

I just realized that the incorrect rank was engraved for Pettifar; it should be guardsman, not private. The other three medals also have pte. as his rank.
Surprised that was accepted by him, or was...

While it was indeed policy not to name WWII medals, many were privately named, especially Indian Army medals, and, in the UK, Boots the Chemists offered medal naming services; I have seen a number of...