Why fast battleships did not require all-forward guns

Would an earlier Royal Navy fast battleship have pushed later battleships, battlecruisers, and cruisers toward all-forward armament sooner?

Not necessarily. The long Admiral-class hull showed that British yards could obtain the desired speed without immediately concentrating every turret forward.

All-forward armament was not an automatic consequence of adopting fast battleships earlier. The Admiral-class design was already extremely long because it aimed at very high speed, while a less ambitious fast battleship could fit British infrastructure without needing the same layout shortcut.