Get loft insulation – ensure it is at least 270 mm thick

If you have gas radiators add thermostatic valves

Insulate your hot water tank

Add some draught-proofing

Install energy-efficient lighting

Include a smart meter

Replace an old boiler with an energy-efficient model  

Install double (or triple glazing)

Add cavity wall insulation (if there are two layers of brick in the outside walls)

Add solar panels.