Why Furious's 18-inch gun did not lead straight to Hood

Why did the Royal Navy not continue immediately with 18-inch guns after HMS Furious?

The idea continued in the M3 studies until treaties intervened, while the Admirals used established 15-inch production to speed construction.

The Royal Navy retained plans for 18-inch guns after Furious, including the M3 battleship design named in the answer. Naval treaties then prevented those plans from proceeding. The Admiral class followed a different timetable because the navy wanted to build quickly using an established 15-inch production line. Alternative Admiral studies considered heavier guns and triple turrets, including a fast-battleship arrangement with twelve 15-inch guns at an estimated 29 to 30 knots. Production readiness therefore helps explain the selected armament without implying that the navy had abandoned larger guns.