Why the Royal Navy preferred an 18-inch leap

Why did the Royal Navy consider 18-inch guns instead of moving from 16 to 16.5 inches?

Because 16.5 inches offered too little overmatch, while an 18-inch weapon provided the larger power increase expected for the battleship role.

The Royal Navy wanted to overmatch other navies' guns. Half an inch beyond a 16-inch weapon did not provide enough gain to justify a new calibre. An 18-inch gun made a much larger step in shell weight and fighting power, while British logistics were accustomed to managing substantial calibre changes. A 16.5-inch weapon made more sense for the proposed G3 battlecruisers, whereas the associated battleship concept was intended to carry the heavier gun.