North facing double glazed windows by themselves can only do so much particularly in houses with no insulation in the walls [I'm guessing] and with a frame that isn't thermally broken [I'm pretty certain of this from your description].
If you can buy or get a loan of one of the infra red or remote thermometers from Boots or some other chemist [the battery powered ones that you can stick in a child's ear as opposed to a thermometer], you can go around the house checking the temperature of external surfaces.
From my own experiements an uninsulated wall at 0-4 degrees outside will give around 14 degrees inside with the heat on for a while.
Under the same conditions a double-glazed UPVC window with a roller blind will give 18-20 degrees.
These aren't u-values, just comparisons of how cold a wall "looks" to a thermometer.
The point being that a good double glazed window system with a 1mm canvas blind can perform better than in unsulated 10" [250mm] cavity wall.
Adding secondary glazing could certainly improve the insulation, but could well cause other problems, apart from the obvious one of access.
I don't know who told you replacing a door with a window would require planning permission.
Section 4(i)(h)of the Planning and Development Act 2000 allows a lot of leeway for interpretation and a chat to your local planning officer costs the price of a phone call.
I'd be surprised if replacing the door with a full height window - or even one with a low sill - actually required planning permission.
regardles of that you could in general replace the old door with a uPVC model, assuming the house isn't a protected structure.
But really, it sounds to me like you need to do a lot more than just replace the door to improve the u-value of the property and you could investigate what grants might be available.
One thing is certain, energy isn't going to get any cheaper in the long run, so reducing dependency on it is the best way to deal with future increases in demand.
Can I say that its heartening that so many people are now focussed on this issue as its important on many levels to reduce energy consumption.
FWIW
ONQ.
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