Hello Brian,
My knowledge of British uniforms is scarce, but I have been intrigued by your comments and researched a bit. Hope you do not mind if I share my thoughts.
Seems to me that a second strap for attaching the entrenchment tool is there:

Resulting like this (which I think it is original):
Just the painter forgot to paint it yellow and left is as part of the tool in black.
The helmets as far as I know were one-size so depending on the guy's head it may look bigger or smaller. If according to your experience, it looks wrong when compared with the rest of equipment, I can not argue.
And the anklets I have used (not British) do move, but maybe it was because of my thin legs
What do you think?