Keeping battleships needs a support system

Why not retain captured or foreign battleships after the war?

A navy needs compatible training, manuals, maintenance, and logistics as much as it needs a ship hull.

Hull performance is only one part of the comparison. A retained battleship also needs trained crews, manuals, spare parts, maintenance practice, and a compatible logistics system. Britain could support its own ships more readily because that structure already existed. Keeping Littorio-class ships would make more sense for Italy, which already had the necessary support arrangements.