I can't get exactly 19.2kW, but close enough if I assume you live in Scotland and have single glazed metal windows.Hi 8m, 7m, 4 external walls, 50 mm loft insulation. 10 rads & I can't remember anything else. Thanks
Double glazed UPV windows would reduce the requirement by 2kW and increasing the loft insulation to 300mm would reduce it by another 3.5kW. Lost insulation is a no-brainer as it will pay for itself in a couple of years. Double glazing has a much longer payback time, but would be a selling point if and when you move.
You may be able to get help paying for the improvements through the Green Deal