You probably have plenty of weapons already, but if you need more SMLE rifles, Stokes mortars, Vickers guns, ammo boxes, and picks and shovels, the Airfix WWII British Infantry Support Group has a lot of such things. They are little different from those used in WWI except for a few details. You can save the soldiers, the Sten guns, and radios for other purposes.
The scale is 1:32, but the difference is barely noticeable, the kits are cheap, and the detail isn't bad.
The main explosive used in the tunnels was Ammonal, but I haven't found any pictures of how it was packaged. Perhaps the Imperial War Museum can help. Ammonal was hygroscopic and always packed in tins or rubberized containers, and I've seen references to 50 pound boxes.
With a lot of wood crates on hand, you might expect to see small fires and improvised stoves made from tins for brewing tea and heating rations.
Somewhere I recall seeing an idealized diagram of a trench layout which had a large notch cut into the lip on one side which held an enormous ball of barbed wire called a stopper. It was intended to be pulled down to block the trench in the event of an enemy raid, giving the defenders time to respond.
I'm anxiously awaiting the end result.
Cheers,


Dick