Some advice for you...
You must use a FENSA registered contractor or apply for Building Regulation approval. This is a legal requirement and you risk prosecution if found out, or having an unsaleable house if not. The installer must handle all the paperwork for you. Check they are a member here:
FENSA
Only consider a firm price quotation that includes an insurance-backed guarantee for the whole system - sealed glazing units, frames, installation, workmanship, resistance to fading or yellowing, etc. etc. (and yet more etc.)
Check they have adequate insurance for the work i.e. that your property is covered if, for example, they remove an existing window frame that is load-bearing and the floor above collapses (I am not joking, this has happened far too often).
Make sure their products conform to the Building Regulations (especially Parts L and N), and to British Standards i.e. BS 7412:2007 (plastic windows) and BS 7950:1997 (security). If not, ask why not!
Make sure the quotation is all-inclusive e.g. repairs to surrounding finishes, trickle ventilation, fitting any trim, VAT, etc.
Don't pay anything up front.
Don't take any credit offers.
Ask for local references. Ring them and go and have a look.