I've been through this a few times now on my old flat and my current house.
If you get the likes of Everest around and they offer you a 'order now bonus' DO NOT take it. In this climate you have a lot of power to get the price down. I was quoted £20,000 for replacement sash windows in my previous flat. I said it was too much, eventually the price went down to £5000.
In the end I just replaced 2 broken windows by a company called Zeneth (also known as Staybright or Bowater windows. DO NOT USE THIS COMPANY IN ANY OF IT'S VARIOUS NAMES. THEY ARE AWFUL.
When a double glazing company sends around their 'designer' they mean a salesman. Do not trust them. They will tell you their windows are better. I had all the major companies around and all the windows were pretty much identical.
I had a quote to replace the terrible plastic double glazed windows in my new house. Plastic and wood were the same price.
Negotiate, play one company off another, don't sign anything on the first meeting and don't fall for the old "do you have a postbox nearby, do you have a bustop nearby, a school...good, you now qualify for our bonus scheme, if we can erect a sign outside your property we will give you a 20% discount if you sign today" con. It is nonsense, absolute piffle.
If, as I have, you have the Double Glazing salesman's boss calling you and the salesman won't leave, go to the phone and tell them that if they don't leave you will call the police. It's the only way I could get rid of one tw*t who had hounded me for over 2 hours with his boss constantly calling back with lower quotes and getting angry with me for not signing there and then.
Plastic windows have thicker frames than wood and block a lot of natural light coming into the room, especially on small windows.
Plastic windows don't last as long as wood. Their 'lifetime guarantee' if often only about 20 years...if the company still exisits in 20 years! Check the terms and conditions. Wood can last much longer.
You can doubleglaze exisiting wooden sash windows (try the sash window workshop or similar) It's cheaper too.
Plastic PVC frames are VERY poor insulators. The plastic itself isn't strong so they have metal running through the frame for strength.
Wood is more environmentally friendly as most companies use FSC wood which is beneficial to the environment. Plastic is oil based and is incredibly bad for the environment.
It's cheaper, adds value to your property if you refurbish your existing wooden windows. Many properties have windows that are over 100 years old (like mine did). I refurbished them with thicker acoustic glass.
Many companies fit poor quality plastic windows. The ones in my house creak and groan all the time, don't shut properly, are difficult to close and have trickle vents that you can't close so you get a constant draught in the room anyway! They had a cheap plastic architrave inside fitted by some one armed, blind deaf and dumb neandertal with a leaking glue gun. I'm pretty sure his other arm was a fingerless stump judging by the due care and attention he attributed to the job in hand. Beware of cheap and poor quality.