Since first reading this thread, I have had a shed constructed on my Mothers propoerty.

About 11 feet by 17 feet, built of concrete blocks on a cast concrete base.
Three walls only being required as the structure is a lean-to against an existing stone wall.
The door is substantial hardwood, fitted with a mortice lock and outward opening.
No windows are fitted, ample light being provivded via the roof which is of transparent polycarbonate.

I chose concrete blocks rather than timber for both security and fire safety.
The type of substantial timber construction disscussed in previous posts, is almost unknown here in the UK. The choice is between blockwork or very lightweight prefabricated timber that is very vulnerable.

Fire risk concerns me somewhat since most of the contents are flammable, and the location is in a small town with other structures nearby. No electrical installation of any kind is used, again to reduce fire risk. The shed is for long term backup stocks, not for items that may be required suddenly in the dark