How a 1909 super-steel would change British warship design

How would Britain use a mass-producible steel breakthrough in 1909 that greatly improved structural strength?

Keep the improvement secret and use it to strengthen ships at their existing thickness, including the steel behind armour, rather than multiplying hulls without crews.

The question starts with steel twice as effective, while the answer later slips into fourfold examples. Its consistent recommendation is to keep the advantage secret and make existing hull thicknesses stronger. Destroyers would benefit directly, and improved structural steel could also back conventional armour. Reducing thickness to multiply hull numbers is rejected because that would still require more crews. The qualitative argument favours stronger individual ships, without a consistent numerical multiplier.