A leading navy cannot stop studying the next capital ship

When did the Royal Navy realize it needed to look beyond the battleship for the next capital ship?

It never stopped looking. Maintaining naval leadership required continuous study of battleships, battlecruisers, large submarines such as X1, and aircraft carriers such as Ark Royal.

A navy that waits for a rival to reveal the next decisive type has already surrendered initiative. British battlecruiser, submarine, and carrier experiments were parallel investigations into future fleet power. X1 and Ark Royal belong to that search even though they represent very different solutions. Continuous design work was a requirement of being the leading naval power, not evidence that every proposal would be built.