Could Vanguard follow-on ships use 15-inch guns and turrets recovered from an R-class battleship?
Short answerPossibly, but Britain has plenty of barrels while salvaging complete turrets from a sunken ship such as Royal Oak would be much harder.
NotesThe proposal is technically possible because Britain retains many 15-inch gun barrels. Complete mountings are the scarcer resource needed for a Vanguard-style reuse program. HMS Royal Oak is already sunk at Scapa Flow, so taking her turrets would require a substantial recovery effort in wartime. An R-class source is therefore possible, though much less straightforward than the existence of spare guns alone suggests.