1 April 2018 saw the introduction of the Minimum Energy Efficiency Standards (MEES) for domestic and commercial rental properties.
Any new tenancies must have an EPC grade of E or better. From April 2020 all domestic existing lets must achieve at least an E. Non-domestic date is April 2023.
There are penalties for non-compliance but, also grants for insulation, heating and lighting.
There are exceptions and more detail to MEES; which is linked here and can be explained if you call Mark at Assess Energy Solutions UK
Elmhurst Energy factsheet MEES - Private Rental Sector - Landlords
Elmhurst Energy factsheet MEES - Tenants