Mate whatever you do, DO NOT go into a company and speak to a salesman, you will get shafted well over the odds. They are selling em to you so everyone in the firm gets paid and they earn a commission, trust me ive worked for a firm for 10 years and myself for 2, all i ever hear from customers is how salesman have taken the mick and ive seen a few in action over the years lol.
Unless you know someone who can reccomend a good firm id go for private work anyday, man in a van aint got all the bills that a manafacturer has.
Take into account do you want the basework included or will you get a builder in?
If you get builders in let them finish the base before you get someone to qoute the windows and roof, its far easier to measure from an existing base, and make sure you arent building over drains etc you might need access for.
If you post up a pic of the area you are thinking of plotting it on, and what style you want, i could give you a rough idea of what i would charge you as a privateer.
Sorry im not interested in doing it thou its a bit too far away for me, but i can give you a bit of advice if you need.
IMHO though, find a decent window fitter, ideally someone whos worked on someone you knows house so you can see their work first hand, all depends on size and style as for price, then you can calculate labour on that.
Hope that helps, if not ignore me pmsl