Various ways to skin the cat.
Double glaze can vary in U value hugely, depending on the frame build up and double glazed units used. We have triple glazed windows and 4 double glazed outside door sets -mainly due to weight.
UPVc/aluminium U values came in at between 1.4 and 2. something, which is rubbish. We went for wooden (and all windows) with krypton gas between the double glazed pains, total U value for the door is either 1.0 or 1.1, a while since a looked.
Unless the rest of your house is well insulated and draft proof, triple glazed is long way down the list of things to spend your money on.