Why Britain put a 12-inch gun on a submarine

Why did the Royal Navy build a submarine with a 12-inch gun?

The cruiser-submarine idea promised powerful surprise attacks without relying solely on scarce torpedoes, but it was tactically slow and dangerously exposed after surfacing.

A large gun appeared to solve a submarine's limited torpedo capacity and promised surprise attacks against major warships. Other navies' experiments encouraged Britain to try the concept, and submarines faced no cumulative numerical limit at the time. On paper, several boats could surface near an engaged enemy battle line and deliver 12-inch shells at short range. The plan broke down operationally because a boat could not surface and fire instantly and needed to approach dangerously close before engaging. That delay exposed it to capital-ship secondary batteries and gave screening destroyers an ideal target. An impressive weapon therefore depended on a firing sequence that made its submarine unusually vulnerable.