Why might Vanguard remain a one-off?

Would Vanguard's speed make her especially useful in Europe or the Pacific, and would Britain build more than one?

Probably not more than one; her speed remains useful, but eight 15-inch guns soon look underarmed beside new fast battleships.

Vanguard's speed gives her maneuver value, but full armor does not make eight 15-inch guns competitive with the projected next generation. Britain needs scarce slipways for ships that can answer Japanese 18-inch weapons and the larger British or American responses. Old 15-inch guns may remain useful ashore when mounts permit greater elevation, but the shipboard installations cannot recover the same long-range performance. Vanguard could therefore be completed as a one-off while later construction shifts to 28-knot fast battleships with heavier batteries.