What if the British ships at Trafalgar were replaced by Yamatos with Cold War Iowa-class refits?
Short answerThe opposing fleet would be overwhelmed, although its proposed replacement is unclear in the question.
NotesYamato battleships carrying Cold War Iowa-class refits replace the British ships at Trafalgar in this counterfactual. They would overwhelm and swamp the opposing ships, leaving no meaningful fight. The proposed replacement for the opposing fleet is unclear.