If existing sash windows are very badly damaged or have been removed, you may want to reinstate them.
If the sash boxes remain and are in good condition, some companies can reuse them, saving money, and provide new sashes, with modern draught-proofing technology. A typical cost for this option is £300.
If you need a complete replacement, you can go for wood, uPVC and aluminium.
If your problem is that the windows are in good condition but need more effective sound and heat insulation, then an excellent solution is secondary glazing. For example, Storm Windows offer bespoke secondary glazing which, because of its design, is almost invisible.
There are a number of companies that make and install complete wooden windows; these have top quality wood that is as stable as the well-seasoned wood used in the originals, but they use some of the modern technologies such as nylon sash cords, good quality sealed units, microporous paint which flexes and is easy to refresh every few years, and high security locks. They are expensive but will last many years longer than the cheapest alternatives.
There are a few companies making good copies of sash windows in aluminium; aluminium is a recycled metal and so good environmentally.
Many people choose to replace sash windows and opt for uPVC. While these may be effective against draughts and give some noise insulation, they rarely match the original wood windows in appearance. These modern materials do get shabby remarkably quickly, the springs and nylon fittings break, the sealed units are often poor and suffer from internal condensation. These problems are expensive to remedy.
If you do opt for plastic windows, try to get the best match with the original windows; usually this will mean a central, horizontal bar, with one pane above and below, or perhaps a four-pane window. Diamond-pattern leaded windows are rarely appropriate.