It depends on the company that you go to. You’ll usually have to request a quote from them as it’s difficult to give a price in advance without knowing more details of the project. Factors such as the size of the windows, the style, the finish, the energy efficiency rating, and more will all affect the price.
Where you live will also influence the price as companies in more expensive areas of the UK may charge more to compensate for higher overheads. For example, a sash window company in London is likely to charge more than a similar company in Manchester.
As a rough estimate, you’ll probably be looking at around £400 per window.