Type 45 Duncan gives the interwar Royal Navy an air-defense blueprint

What changes if the modern Type 45 HMS Duncan replaces the D-class destroyer Duncan in 1932?

The Royal Navy gains enough time to study modern sensors, missiles, close-in defenses, and ship systems before the Second World War, radically improving fleet air defense.

The single modern ship would be valuable less for the finite weapons she carries than for the design information available before 1939. Radar architecture, combat direction, missile guidance, and Phalanx-style close defense would give British engineers concrete targets for adaptation. Even partial replication could make the Mediterranean air war much more costly for Axis aircraft.