Improving the energy efficiency of historic buildings is an important issue but up-grading work needs to be done with care. The way forward is to follow independent and informed guidance such as that provided by Historic England or the Society for the Preservation of Ancient Buildings. Finding the correct advice in an easily understandable way takes time but well worth doing to prevent expensive mistakes, and beware of adverts that make overambitious claims.
For advice from Historic England
https://historicengland.org.uk/advice/your-home/saving-energy/making-changes-to-save-energy/
and from the Society for the Preservation of Ancient Buildings
https://www.spab.org.uk/advice/energy-efficiency-old-buildings