Ex-council flats are an electric heating problem, not an insulation one
A great many purpose-built flats round here still run on old electric storage heaters, and that single fact usually matters more than the walls, the windows and the loft combined. Electricity carries a much higher cost and carbon weighting than mains gas in the calculation, so an otherwise sound flat can sit at E or F on heating alone. Replacing ageing storage heaters with modern high-heat-retention units is normally the biggest single move available.