A future Type 48 railgun destroyer

Would the Royal Navy call its first armored railgun cruiser a battlecruiser or battleship?

It would probably be called a destroyer and treated as a Type 45 successor, regardless of cruiser-like scale or armament.

A railgun successor to Type 45 would probably be described administratively as a destroyer even if its displacement and weapons resembled a cruiser. Following the gaps between Type 42 and Type 45, Type 48 or Type 49 are offered as plausible project numbers. Classification would reflect institutional lineage and political presentation more than a strict historical tonnage rule. A tumblehome hull could also become easier for the Royal Navy to accept once other navies have normalized the form.