Firstly, you cant fit a 'proper' roof as the structure is not able to take the associated loads.
Generally what appears to be done is that Celotex (or equivalent) is fitted to the under side using wooden battens and then plaster boarded over, some also then add a light weight roof covering (shingles or similar, I Have seen concrete tiles used but these would be way to heavy, even with Marley lights).
I do have some concerns with this as the upper side of the insulation will attract condensation and there is no ventilation in this part.
Either way, no matter what you do its going to be a compromise.
(I to hate conservatories, a plastic shed with lots of glass which still has to have an external door between it and the house its to big a compromise)