Why a post-war Vanguard successor needs a new gun programme

Would an upgraded Vanguard remain the likely British battleship design after the Queen Elizabeths were scrapped?

Probably not, because repeat ships would need a new gun programme and a 16-inch Lion derivative offered a more sustainable path.

Vanguard could exploit stored 15-inch barrels and turrets, but repeat ships would exhaust that shortcut. Britain would then have to restart 15-inch gun production, reuse unwanted 14-inch weapons, or revive a 16-inch programme. A Lion derivative offered a more sustainable path because its 16-inch weapon could use common ammunition with American designs. The constraint was the weapons supply chain rather than the ability to draw another hull.