I'm not saying accept, I'm saying that I doubt double glazing will fix the problem of a cold house or flat! I would argue it is NOT the most cost-efficient way to spend thousands and thousands of pounds! Better to use that money to make the kind of 'mini-improvement's listed (eg, thick curtains, DIY 'internal' double glazing etc), and to pay for extra heating of the existing system, assuming that that is working at capacity.
It's easy to tell how 'naturally cold' a house is if, like me, you have built in wardrobes. If I've got the heating on in the house, and then open the wardrobe door, the air inside is far, far colder, and yet there are no windows, double glazed or not, in the wardrobe.
It would be interesting to see a quote for double glazing of your property, and then your heating bills, and see how much extra heating the cost of the double glazing would cover.