Starting from scratch, insulating your loft and cavity walls could save over a tonne of CO2e and reduce your heating bill with a relatively small outlay.  

However if you already have a certain amount of loft insulation and are just topping up to the necessary 270mm thickness, the savings will be much lower as the orange bars above reveal.  

Cavity wall insulation also achieves great savings in CO2e and heating costs with a relatively small investment. There are also grants for this in some areas. 

Pre 1920s houses are generally solid walled and so not suitable for this form of insulation. Check out solid wall insulation in the case of older houses. 

If your house is brick built, you can tell if you have cavity walls if you can only see the long side of the bricks. If short sides, or ends are seen in the body of the wall, your walls are solid. 

Cavity wall brickwork.       

Solid wall brickwork. 

Photos courtesy John and Nick Cowie under creative commons licence.