Check them out. Some firms have been going since Ted Moult, and look like being here for a while. Whatever, ensure they are members of the Glass and Glazing federation,British plastics federation,Federation of master builders,Consumer protection association,Guild of master craftsmen,European builders federation, or as many of the above as possible.That the quality standard is BS570 and that they have public liability insurance. A certificate should be issued after completion,ie FENSA,CERTASS or British Standards Institution,to show that the work complies with the regulations.
Ask will they achieve a "u" value below2.0W/m2K, (the lower the "u" the better,Low"E" (glass, like Pilkington"K") a between glass gap size of 20mm. Are all seams fully welded? Will guarantees be in writing? ie glass type, making good soffits etc? Will they have equal site lines,gasket colours ,locks.
All uPVC frames are,not as mentioned, the same, the cross sectional areas vary some have more metal, and are stronger,and are better at supporting a wide window gap.
what are the guarantees Are you insurance backed,,how long are the units guaranteed for?( nothing worse than misted up units). You may well not get all the above, but many are essential and are a must.See if you have any local work that has been carried out by the firm you are considering.
Hope this is of some help.
ps, How long will it take?