Yes, it's a great photo, the rider looks happy and pleased to be photographed on the BSA. The bike looks to be in quite good nick and doesn't appear to have seen much front line action, I wonder if he is one of those riders who joined up with their own bikes, maybe that's why he's a sarge!
Could be wrong but I thought WD issue BSAs were all chain drive so this could also point to a private owner machine. Imagine using that BSA with its zilch ground clearance, belt drive, and skinny beaded edge tyres in the conditions riders had to endure in WW1, they had to undertake journeys of two hundred miles in some cases, not just short runs behind the lines. Also carried baskets full of messenger pigeons on the rear carrier, I bet some of those ended up in the pot.
Let's hope this rider and his BSA made it through the war.
BT is a Yorkshire registration so I'd think almost certainly not a WD machine. The series ran though from 1903 to 1926 which doesn't help us to pin anything down.
The art-nouveau tiling is interesting. It could be France...but the ventilation grilles look more English. Maybe a theatre or something ?
I thought the background looked French, didn't realise the reg was Yorkshire though, dim of me, thats where I live, so may be taken on home ground and maybe the bike is not on active service. Those tiles look a bit posh for Yorkshire though.