Make sure the company is FENSA registered - this means that you will not have to get the work signed off by the Council under the Building Regs.
http://www.fensa.co.uk/homeowners.html
I have had two lots of windows in my present house - the first was hugely expensive by a 'big name' and nothing but trouble from the start until I had them replaced by a small local company. What a difference! Fantastic job done three years ago.
So, don't be swayed by big names. Get at least three quotes. If they tell you you can have a huge discount if you sign NOW show them the door. They are giving you the hard sell and ripping you off.
Don't be bamboozled by technical jargon that means nothing to you.
Ask people for recommendations.
Where do you live, by the way?