Why obsolete battleships make poor reserve ships

Should the Royal Navy have kept its older 13.5-inch battleships in reserve?

Generally no. Their fuel, speed, and modernization problems would cost too much for ships still vulnerable to newer battleships.

Reserve value depends on ship type and mission. Older destroyers and frigates can still escort convoys or perform second-line work. Thirteen-and-a-half-inch battleships, however, impose large coal, crew, and modernization burdens while lacking the speed to evade modern opponents. Once enough money is spent to correct those weaknesses, the navy is effectively rebuilding the ship, erasing the supposed economy of retention.