All window installations must comply with building regulations and the route to compliance which has the lowest up-front cost and simplest paperwork trail is the whole value scheme through FENSA. Simply specify a low-E glass, a warm edge spacer and a gas filled unit and the insulated glass (IG) unit will be deemed as complying with the 1.4W/m².K whole window value for PVC and Timber frames of a minimum of 70mm thickness. 

In the long run, WER ratings available through BFRC, CERTASS, BSI and other competent persons schemes are well recognised and highly marketable, so it's worth exploring this option. The process depends on whether you buy Glazed or Unglazed frames.