The energy companies were required to provide free and discounted energy saving measures for their customers, to a certain value each year. This was as part of the overall carbon lowering measures.
They typcally provided loft and wall insulation, both of which could be done in a day. If you had solid walls it took a little longer and some properties weren't suitable.
For the vast majority it was quick, easy and cheap. No major construction required either for most.
I've actually done a lot with my house when I've been having construction done anyway but there's no easy way to retrofit insulation into the existing walls. (Plaster lath etc.)
They can ensure that every house sale includes the information on how well insulated a house is, and those who like to be cosy or save money or the planet can buy a different home.
No one's forcing you to do anything, they may however object to you forcing them to pay for your extravagances via market externalities.
The house I live in is almost 200 years old. What sort of regulation and government intervention do you have in mind? Should everyone with a house that's not up to the latest modern insulation standards be forced to undertake a major construction project to update all their walls and windows?
As with many things, the issue isn't with the technology and standards that exist today. It's with the large installed base that can't be just wished away.