If you look carefully at the fourth photo, you can see where it looks like another cap badge has been fitted before. There appears to be two holes in the cap-band under the existing badge.
Also, I would go with the general concensus that the cap-badge is probably Belgian or Dutch.
Having just blown up the photo's, the side buttons appear to be British GS buttons.
As usual, I am willing to stand corrected by those with more knowledge than me.