How thick could British foundries forge battleship armor?

What was the maximum thickness British foundries could forge for battleship belt armor in this era?

About 16 inches, comparable to the maximum thickness used for turret armor.

British foundries are credited with the ability to forge battleship belt plate about 16 inches thick in the period of the Agincourt design exercise. The figure is described as approximately the same limit as turret armor. It provides an industrial basis for the reconstructed ship's 16-inch belt, without a further distinction between armor grades or manufacturing processes.