Heating the home is the largest source of carbon emissions in the UK.
By heating the home with gas, electricity, or oil, we are directly burning fossil fuels which contribute to global warming.
Many of our homes are badly insulated which requires more energy.
And many of us keep our heating unnecessarily high.
So turning down the temperature by a small amount, and insulating the home are small ways to make a big difference to the climate (and your spending).

The Energy Saving Trust has lots of useful information on reducing carbon in the home:

Top 10 Energy Saving Measures