This is one of the most cost effective measures you can take, starting with draught excluders, fitting individual room radiator thermostats and a minimum of 200mm of insulation in your loft (there are often grants available to help with this).

Next on the list should be checking your windows. Simply fitting double lined curtains can make a big difference in winter, but double glazing is regarded as a minimum standard. If you have single glazed windows that are not suitable for replacement (sash windows in a period property for example), then consider fitting secondary a secondary glazing solution, which can reduce energy loss by up to 70%

Next is floor and wall insulation, but you are usually best starting with a professional assessment of our property as the high investment cost versus energy saving benefit is only woth it if the work is done effectively. 

This is one of the smartest changes you can make and the positive impact is huge.

Renewable energy from wind and solar is now the UK's cheapest form of energy generation, and by purchasing it through a verified supplier, you are helping to accelerate the overall market provision, ultimately leading to a more secure, cost effective, low-carbon energy sector.  

There are lots of options, but look for verified 100% renewable from suppliers such as Good Energy, Ecotricity and Octopus. 

You could also consider investing in a community project, some of which mean that you will get a discount on your bill.