Capital ships become siege engines

In this counterfactual, construction of a German battle fleet would itself look like preparation to retake Helgoland, encouraging Britain to reinforce the island further. Submarines, torpedo boats, destroyers, mines, and coastal guns would give Germany a defended barrier to breach before the main fleets met. German capital ships would become siege engines built to survive the island's defences and force access past them. Speed matters less because the objective and the likely battle area are already known; armour, subdivision, and firepower determine whether the ships can endure until the Royal Navy arrives. Britain would still need globally deployable capital ships and could outbuild Germany, so the specialised fleet would have to impose losses large enough to force British design compromises. The fixed geography gives Britain the option to reinforce on its own terms unless Germany first accepts the island's attrition.