That wasn't a Webley Mk VI in the movie, I don't believe.
The Mark VI had a square butt, and I distinctly recall a bird'shead grip, which would make it more than likely a Mk. III or IV.
The Mark V was far too late, having been adopted in, I believe, 1915 or so, with the Mark VI very shortly afterwards.
In any event, a Webley of any kind wasn't proper.
The Webley Mk I was adopted in 1887.
I'm not certain, but I don't think the .476 cartridge was adopted until after Rorke's Drift, either...