Why Britain did not reserve 9.2-inch cruiser turrets

Should the Royal Navy have retained 9.2-inch turrets for a later large cruiser?

No; newer plans existed, and wartime standardization favored weapons already ready for rapid production.

The proposal would have preserved old heavy cruiser weapons for a hypothetical 15,000-ton County-type ship. Britain already had satisfactory newer gun designs available. The decisive constraint was wartime production, when standardizing around designs already in manufacture mattered more than reviving stored weapons. Armament choice therefore depended on industrial tempo and ammunition commonality as well as gun performance.