You shouldn't need a specialist conservatory installer, most small builders deal with conservatories all the time.
Personally I would recommend a tiled/slate roof (to match existing house) rather than glass - it keeps the heat in in winter, and allows the low winter rays into the house, while at the same time preventing the place from overheating in summer.
If there is slightly limited light, then also consider veluxes.
Be aware of north and the direction of the sun around the house - especially western light which would be the most important for dining areas (barbies in the setting sun etc.)
Be aware of the best access to any patios - and integrate the conservatory with any patios and further into the garden.
Also be aware of any access requirements from the kitchen to the utility to the outside.
Talk to your local Planning Authority (or check out online) regarding Planning exemptions - it is 40 sq. m., but there are other qualifications involved as well so make sure you qualify under all headings.