Polycarbonate conservatory roofs age badly. Within 8–10 years the sheets yellow, algae grows between the twin walls, and the noise of rain becomes unbearable. In summer the room is a greenhouse; in winter it's colder than the garden. Condensation drips from the underside every morning.
Replacing the whole roof with a solid tiled one costs £6,000–£15,000, needs planning permission in most cases, and takes weeks. For most people, that's not on.
A PVC ceiling fitted underneath the existing polycarbonate does most of the same job for a fraction of the cost — waterproof, insulating, and it hides the yellowed roof completely. No planning permission, no builders, no scaffold.