Hi Brock
The pump shotgun was not issued in the British Army and I don't know of any pictorial evidence to confirm the use of pump scatterguns by the British, although all sorts of wierd, sometimes medieval weapons were in use for raiding. I think it unlikely that they would have managed to get their hands on them, even though they would have proved useful. A lot of non-regulation weapons were in use, chiefly by officers who would have provided their own via private purchase, and no doubt a few double 12-gauge sporting shotguns got out to France and into the trenches, but likely not into the hands of the PBI
Just trying to think what you might replace the Lewis with..... how about the standard SMLE with a cup discharger for Mills grenades ... a bit different.
Let us see the results when you have decided what to do with him
Cheers
Phil