Look at pictures the pipe has 43mm for diameter stamped on it. It' way too large even for a shotgun shell. This is the police just being idiots or saving face. This guy really needs a better lawyer. I am honestly amazed a judge would not even question it.
I bet it was just a container to keep stuff dry from the moisture in the dirt since it sounds like this guy hid stuff in the ground in caches.
The Firearms Act does:
http://www.legislation.gov.uk/ukpga/1968/27/section/57
Whether or not the firearm is offence worthy:
http://www.legislation.gov.uk/ukpga/1968/27/section/5