The guidance issued by the Department for Business, Energy & Industrial Strategy on this subject is very clear. It states:
There are no specified materials or improvement measures; a landlord is free to do whatever they like with their property so long as the EPC rating can be raised to meet the Minimum Energy Efficiency Standard. The most assessable source of advice would be the recommended measures section on EPC for the property, but landlords can seek advice from other suitably qualified experts if they wish.