Aside from having your windows/doors replaced by a company that is a FENSA member, many companies will sell energy rated windows/doors. Find a FENSA member and then check to see if they sell energy rated windows/doors as a BFRC authorised installer here.
These are rainbow labels that are similar to those found on fridges, freezers, washing machines.
They take into account the whole window or door - not just the glazing element because all components are incorporated into the overall calculation to correctly identify how energy efficient a product is.
The British Fenestration Rating Council (BFRC) is the premier UK authority for independently verified ratings of energy efficient windows and energy efficient doors.
To learn more about energy ratings on windows and doors or to locate a double glazing company who is able to supply them visit the BFRC website