If you already have a conservatory then the best option would probably be a fully insulated and tiled roof but almost all conservatory frames and foundations would be unable to support this extra weight. A rather extreme option would be to demolish the conservatory and basework and rebuild the whole thing including more substantial foundations.
An alternative to this would be to have a ceiling installed, BUT BEWARE!!
There are numerous companies across the country to offer this kind of service. The general practice is to fix timber battens to the existing rafters, fit some insulation between the timbers and fix plasterboard to the timber and skim with plaster over this then decorate. This on its own is a lot of extra weight for the roof structure to carry (bearing in mind it was originally intended to support only lightweight roof panels) as well as misplaced screws making holes in the roof structure. This could cause stability issues with the roof and/or the frames beneath. Also what happens if the roof leaks at some point due to the additional weight or the polycarbonate becoming brittle as it gets older? The first you will know about it is when the ceiling becomes ruined.
Conservatory Climate Solutions Ltd are an authorised distributor and installer of two different insulated roofing systems; Thermotec and Ecotherm insulated roof panels to ensure your conservatory is a room that can be used every day of the year, whatever the weather.
Both products are lightweight in comparison to other alternatives available so they do not impact on the stability of the roof structure.
Thermotec Insulated Conservatory Roofing System
Read more about the Themotec Conservatory Roofing System
Ecotherm Clip-in Insulated Roof Panels
The Ecotherm option is a more affordable insulation system that clips in to the inside of your existing roof without any structural alteration and without the need to remove your existing roofing materials (glass or polycarbonate).
Read more about the Ecotherm Conservatory Roofing System