Wahey, it's the Firepower Demo in Warminster from four days ago! (Or, at least it looks like the same guy)
It's a Benelli M4 shotgun, capable of both pump-action and auto operation, according to the announcer it was introduced very recently.
The chap standing is (I think) the section commander of 8 guys from 3 Mercian demonstrating what your average rifle section looks like. I was surprised to see Osprey body armour in Britain - I thought it stayed in-theatre? - as I was also surprised to see him trying to balance his rifle upright when holding up the shotgun and it falling about in the process to much merriment.