The Admiral class changed because the war changed

Does the Admiral class show that Britain did not know what it wanted from a battlecruiser?

No. Britain had a clear initial requirement, then changed it as new wartime evidence altered what a viable ship needed.

The Admiral class was not born from an absence of requirements. Designers knew what they wanted when the ships were ordered and began construction against a coherent contemporary threat picture. Wartime evidence then changed that picture while the hulls were being built. The resulting redesigns show an effort to make the ships viable in a newly understood environment rather than proof that the original staff had no idea what role they needed.