Store the calibres a future force can actually use

Would it be worth stockpiling 13.5-inch ammunition for a rapid rearmament programme?

No; the better reserve is ammunition for weapons with plausible later service, with 15-inch guns and common medium calibres offering much more value.

Preserving 13.5-inch ammunition solely because it belongs to famous ships has little value. The suggested priorities are calibres with plausible future roles: four-inch, three-inch, pom-pom, and six-inch weapons for escorts or local defence, plus selected 15-inch guns for heavy coastal or capital-ship use. Ammunition value depends on having compatible weapons, mountings, maintenance, and an identified wartime task. The reserve should follow a future force plan rather than preserve every obsolete calibre indiscriminately.