Britain's response to a German 21-inch battleship

How would the Royal Navy fare if Germany deployed a radar-equipped battleship with 21-inch guns instead of Bismarck?

Britain would respond years before completion with numerous 16- or 16.5-inch battleships, continued carrier construction, radar, and weight of force; the German industrial buildup could not remain hidden.

The historical Royal Navy would not wait unchanged for a German 21-inch battleship. British intelligence could observe the new docks, factories, proof facilities, and construction from the programme's 1935 start. Britain could answer with proven 16-inch or 16.5-inch guns, more barrels, radar, and carrier strikes instead of matching calibre. The German ship would require so much visible infrastructure that it would telegraph the threat years before completion. Such an extreme breach of treaty expectations might also provoke intervention while Germany remained militarily weak.