Why an early U-boat buildup would not guarantee British starvation

How early would Germany need to replace its surface program with submarines to starve Britain?

So early and on such a visible scale that Britain could counterbuild escorts and exploit Germany's one-dimensional fleet before the campaign began.

Before the First World War, producing the proposed numbers would require starting before practical submarines were available. In the interwar version of the scenario, a conspicuous German submarine programme would invite a British response. Article 8 of the London Naval Treaty is identified as allowing unlimited construction of small ships within specified design limits, giving Britain scope to build more escorts. Abandoning Germany's surface fleet would free British capital ships and carriers for the Mediterranean and Far East, while sloops and other anti-submarine forces dealt with Germany. Italy would then face more British heavy units, reducing the attraction of an alliance with Germany. British intelligence is expected to detect the shipyard activity, and the roughly 300-boat requirement would demand much higher production well before the war.