With the changes that are currently underway concerning energy efficiency, I started to look at ways of improving heat retention. Most heat is lost through poorly fitted or old windows.
With that in mind I started to look at window companies. Yes, you could replace with UVPC and your energy rating would improve. But, wouldn't you like to have an installation that would need no maintenance, that didn't warp, swell, flake or need sanding and painting, and that they would feature the latest security innovation.
You should be looking at installing Residence 9, they are not the most budget friendly of windows, but we all know you get what you pay for. If your home is in a Conservation area, or if you have an older period property I would urge you to look at their website as they make a product that is showcasing British craftsmanship at is best.
Their window system has been designed to look the same as a 19thC Sash Timber window, but having said that they also make windows for more contemporary properties.
If you want to feel safe in your home and you do not want outside noises spoiling your relaxation read up on their products and make up your own mind.