Singapore had heavy guns but failed to secure the water supply

Why did Britain not use a preserved 13.5-inch dreadnought as a museum ship and defensive battery at Singapore?

Because Britain could legally install purpose-built 15-inch coastal guns; Singapore failed through incomplete support and land defense, not a lack of nominal heavy-gun caliber.

Britain had no strategic need to preserve a 13.5-inch dreadnought at Singapore because treaty rules allowed proper 15-inch coastal guns there. The real failures were incomplete batteries, inadequate ammunition, weak secondary positions, and the failure to occupy defenses around the reservoir. Japanese forces could therefore seize the water supply despite the fortress's impressive heavy guns.