Why was HMS Royal Sovereign in such poor condition when the Soviet Union returned her?
Short answerThe Soviet Navy lacked the facilities to maintain a British battleship, so equipment that stopped working was difficult to restore and the ship steadily deteriorated.
NotesOperating a ship is different from sustaining it over time. Smaller vessels transferred to Soviet service could be kept useful, but a battleship required specialized dockyard capacity, spares, and routine attention on a much larger scale. Royal Sovereign moved little, and neglected mechanisms became progressively harder to recover once they seized or failed. By the time she returned, the Royal Navy judged the ship suitable only for disposal. A transferred capital ship is not a usable capability unless the recipient also possesses its maintenance system.