The Renewable Heat Incentive has now closed to new applications and has been replaced by the Boiler Upgrade Scheme which gives a one-off grant that helps to fund Air Source Heat Pumps, Ground Source Heat Pumps and Biomass Boilers.

https://www.gov.uk/guidance/check-if-you-may-be-eligible-for-the-boiler-upgrade-scheme-from-april-2022#eligibility