An H-45 threat would trigger ships and air attack

How would the Royal Navy respond if the fictional German H-45 super-battleship were actually built?

Britain would first try to stop completion with bombers, then accelerate a larger Lion-derived 18-inch-gun battleship if the threat survived.

The first likely British response would be an attempt to destroy the German hull before completion. If air attack could not stop it, an emergency British capital-ship program seems likely. The likely design would extend the Lion class and revive the 18-inch/45 weapon as the largest practical British gun. Aviation, construction time, stored technology, and fleet design would all shape the response.