Probably a Territorial battalion or one of "Kitchener's New Army" battalions, wherein were squandered 150,000 or so of the best, whose absence would become painfully noticeable in 1917 and 18.
And of course ever after!
The trench looks like an early one, before the lesson of bays and switchbacks was learned. Photographers had a freer run of the front in 1914 and 15 than they did later. Another shot taken at the same time and place.
https://www.milsurps.com/images/impo...trench_2-1.jpg
