You will no doubt get a load of polarised opinions.
The purely rational might try and explain that walls, being a large surface area of the total through which heat can escape, will therefore slow the inevitable loss of warmth from the inside to the outside.
However, in terms of cost versus the relative benefit, first make sure you have at least 250mm (and preferably 300+mm) of fibreglass or its equivalent in your loftspace and spend money on draft-proofing properly.
Then consider cavity insulation (before any thoughts about replacement windows and doors - unless they are really old crappy metal framed ones)