sorry cant help with names, however i wouldnt go to a big company, i would look at small local independants mate. the good decent ones will have the same quality as the big boys but at vastly cheaper prices.
ignore the big boys who go on about A rated glass etc. its the law now that all new windows have to be A rated.
white frames are the cheapest, yopu pay a big premium for wood effect or coloured frames.
there is no such thing as BOGOF or free fitting. any one that offers that has increased the prices in the first place.
sometimes its cheaper to go an independant just for the windows and then pay a fitter to fit them.
i know down here its roughly
£120 to fit a door
£80 to £100 for a window(depending on size and ease of getting old one out)
some fitters will give a discount if there are maybe 10 plus doors/windows to fit.
last bit of advice, which im sure you know anyway is to get at least 3 quotes for the windows and the actual fitting. most companies do free measuring and if they then do the job, any mis measures is down to them and they have to replace free of charge. you do your own measuring and down to you if you measure wrong.
ask your neighbours too for recommendations mate.