The government is inviting applications from up to 600,000 homeowners or landlords to be given out in grants of up to £5k covering up to 2/3 of the cost of home improvements to increase energy efficiency or supply low-carbon heat. See
You have to find a registered installer via TrustMark at https://www.trustmark.org.uk
Another useful site is
https://www.simpleenergyadvice.org.uk/
which will recommend local installers.