The Admiral class lacked a convincing opponent

Was the Admiral-class battlecruiser overdesigned?

Yes. Its design continued to answer a threat that had disappeared, while the likely Japanese and American opponents did not clearly require so costly a ship.

The Royal Navy knew German capital-ship construction had stopped, leaving Japanese Kongos and planned American Lexingtons as the plausible comparisons. The Admiral class carried more gun and protection than the anti-Kongo role required, while a Lexington would already face a dangerous fight against Renown.