Why the Royal Navy adopted a 120 mm gun

Why did the Royal Navy use a 120 mm gun instead of an even imperial caliber?

Because it was a powerful, capable gun that worked well; performance outweighed a tidier caliber label.

Caliber neatness mattered less than fielding a weapon that met operational needs. The 120 mm gun was powerful and capable when introduced. Once it performed satisfactorily, changing caliber merely to reach a round imperial measurement offered little benefit. The answer treats demonstrated performance as a stronger basis for standardization than naming convention.