If it's a conservatory you're thinking of, do the foundations yourself or get quotes from local builders and then try Winstone Windows over near Hartcliffe, that's where I get my doors, windows and conservatories from, they are usually cheaper than the others unless you source it from the net but you don't know what quality you'll get.
Extending any space using glass and plastic means it will be unusable for 6 months of the year, unless you chuck money at it keeping it cool or keeping it warm.