You don't say whether you have sashes or casements and whether you want wood or PVC replacements.

We had ours done, reluctantly in PVC, by these guys: Crestshield and would recommend. A family firm, based in Penge.

At first we were going to rule them out because their sales rep (son) failed to keep appointment, but dad chased up and pleaded for a second chance. They especially did a particularly job of replicating the look of the stained glass fan lights in the replacement PVC windows.

They have a variety of ranges and will quote a cheaper window for e.g. a BTL landlord than for a homeowner. You need to be clear about what you are getting for your money and that quotes are like for like.