Why the 4.5-inch upper-deck mount kept slipping

Why did the Royal Navy take so long to provide a high-angle replacement for destroyer 4.7-inch guns?

The 4.5-inch upper-deck mounting entered development late, was repeatedly deferred in pursuit of a better solution during wartime, and did not mature until the Daring class.

The Royal Navy responded to the immediate wartime problem by replacing an after 4.7-inch mounting with a twin four-inch high-angle weapon. Its preferred 4.5-inch upper-deck system was still under development when war constrained engineering time and money. The promised improved version was always six months or a year away, encouraging further delay instead of fielding a simpler mount. The concept finally appeared in mature form with the postwar Daring class.