I should think that there would be little or no saving in gas by doing what you suggest. The boiler will be working for longer to get the temp up to 20 in the house.
I would have thought that you would be better putting thermo valves on your radiators and only heating the parts of the house in use.
I should point out that we have just completed changing all of the casements in our house to new, with the Pilkington K glass double glazing and our thermostat in the hall is now set on 17 degrees.
Got what I thought was a good deal for, for this stage,54 wooden casements, two new hardwood sills fitted and primed with all new window furniture. Total cost was £5400 (GBP)
Local company who have done work for us before, notably replacing a bay window complete with double glazing as above. Bay comprised softwood frame with hardwood sill and 14 casements in softwood with K glass as above. Furniture as above and painted.
Cost was just under £2000.