Why wartime Britain kept the 15-inch production line

Why were 18-inch-gun Admiral-class proposals rejected?

Wartime Britain needed rapid output from its established 15-inch production line; starting an 18-inch line imposed delay and industrial risk.

Britain already had 15-inch production operating and needed ships quickly. Building 18-inch guns required mobilizing a different production line, proving barrels, and accepting delay during an uncertain war. The initial handful of 18-inch barrels did not provide the sustained output needed for a class of capital ships. Admirals armed with the established weapon were therefore the lower-risk wartime choice. An Argencourt already in service would change that calculation by preserving experience and infrastructure for subsequent large-calibre construction.