Elgin Cathedral -:
How does a free standing steel frame with a proper tiled roof, and a central clad tower with a lift grab you? Framed glass entirely where there is nothing at present. Leaving the old building almost as it is and completely visible. Except with a suspended floor off the steel.
Walkways around the roof edge for maintenance, and easily accessing the west towers.
No? Cheap though. Around £5 million or less. Also it would not affect existing features. Windows with curtains. Picture it with just the steel in place , , , ,