British cruiser design began with the role, then defined the target

When Britain designed cruisers, was 'what is this ship for?' the first question?

Broadly yes, but the more important design question was what specific operational standard and threat the cruiser was being built to meet.

The Royal Navy generally understood what cruisers were for before beginning a design. The decisive question was what concrete opponent, route, endurance standard, and operational condition the new cruiser had to meet. The Type 26 became clearer as it developed into a defined frigate. The Littoral Combat Ship followed the opposite path, with a novel label and concept advancing before the operational target was settled.