Headlights on vehicles have switched from glass to polycarbonate or plastic, one reason is simply weight, polycarbonate is lighter than glass which means vehicles become more fuel efficient as they are lighter. The downside to polycarbonate headlights is that they only tend to last around 5 years before they start to go dull or cloudy due to the UV (ultraviolet sunlight).
Dull or cloudy headlights can become dangerous when driving at night due to lack of light filtering through the lens. They could also be a fail on your MOT if they are in really bad condition.
G & G will restore the lens and add the UV protection which will give the polycarbonate the protection it needs so they wont go dull or cloud over. Dull headlights can be a fail on your MOT, if your headlights are not UV protected, then they will go cloudy again.