Why a 6-inch heavy-AA battery was unlikely

Would wartime experience push the Royal Navy toward long-barrel 6-inch secondary guns for high-altitude anti-aircraft fire?

Probably not. The 5.25-inch gun was already in development, which makes a new 6-inch heavy-AA weapon an unlikely change of course.

The proposal combines medium and heavy anti-aircraft weapons with a long-barrel 6-inch secondary gun intended to engage bombers as well as cruisers. The Royal Navy was already developing the 5.25-inch weapon, which makes a shift to 6-inch heavy AA doubtful. Existing procurement work constrains what a navy can plausibly adopt after combat reveals a new requirement. A counterfactual design therefore begins with weapons already moving through the development system.