Which navy has achieved a confirmed submarine-against-submarine sinking?
Short answerThe Royal Navy, although the exchange does not specify what makes the submarine-against-submarine sinking unique. The nuclear-powered submarine claim is explicitly tentative.
NotesThe question credits the Royal Navy with a unique confirmed submarine-against-submarine sinking, and the reply accepts that claim. Neither submarine is named, and the conditions that would make the sinking unique are not specified. A separate, explicitly tentative claim gives the Royal Navy the only ship sinking by a nuclear-powered submarine. The brief answer supplies no further details for either action.