Hi,
We added a 18ft by 17ft sunroom 3 years ago. Roof fitted with vittral window in the roof (a bit like velux), roof to floor glazing, underfloor heating. The cost to build at that stage (we already had foundations) by a good local builder was E23,000 this included all the steel frame, E17,000 for Marvin windows which are expensive. By the time we tiled it and painted it add on another E4,000. It is a lovely addition to the house. I'm also in the South East, we used the builder that built our house but I don't think they are interested in small jobs any more. I think you'd be better going for a building contractor to build, order the windows yourself there are plenty of choices out there now and don't forget to get Certificat of Compliance in case you ever need to sell. Best of Luck