Why Arore would be a fast battleship, not a battlecruiser

Would an eighteen-inch Arore followed by larger Admirals make Arore the Royal Navy's last battlecruiser?

No; battleship-grade protection and subdivision would make her the first generation of fast battleship.

The hypothetical Arore is a six-gun, roughly 28-knot first-generation fast battleship, followed by larger Admirals with eight eighteen-inch guns. A true battleship treatment would require deep armor coverage, multiple protected layers, strong decks, and dense internal subdivision rather than merely a thick headline belt. That structural standard, rather than the ship's speed, keeps the design out of the battlecruiser category.