Why Vanguard would reject American 16-inch guns

Would the Royal Navy accept American 16-inch guns for Vanguard-class battleships?

Probably not; the missing twin turret and the redesign needed to fit it would defeat Vanguard's purpose of reaching service quickly with existing British 15-inch guns and rebuilt mountings.

Vanguard's program relied on guns already in stock and turrets that Britain could modernize. American 16-inch weapons would require a suitable twin turret and major integration work, delaying the ship considerably. At expected Royal Navy battle ranges the practical gain over the 15-inch/42 was not large enough to justify losing the schedule advantage.