My grandfather was in a highland regiment before and during WWI. Kilt aprons were worn over the kilts in the trenches, during training and working parties, but those lads aren't wearing them. I suspect the tartan is a dark on dark one, i.e. Black Watch or similar. None of them is wearing a hackle either. The Lovat Scouts wore checkered band Balmorals so it is unlikely they are Lovats.