I visited a Victorian end of terrace house in London Borough of Waltham Forest fairly recently.
This had been acquired by WF Council and improved with every energy saving method they could think of.
Lots of people visited using (free) tickets. They did explanatory tours. The energy improvements they did included:
Under the floorboards spray on insulation (done by a small robot car ) - they had to cut a 30x30cm hole into the floor board to initially place the sprayer. It was then guided by computer from the room. Ground floor only.
A heat source pump in the back garden.
Internal insulated plasterboard (so slightly smaller rooms).
They kept the old glazing as it was reasonable but not worth replacement.
Solar panels on the roof at r back of the house and part of the kitchen made into a room for the giant batteries to store energy.
Something under the bath tub to remove and reuse the heat that old bathwater has before it goes into the drains.
There was a multitude of other stuff too.
They spent £67,000 on energy saving improvements in this conservation area 4 bed house.
The Council plan to let a local family rent it and will monitor energy usage.
It was really interesting to see and hear how it can be done, but how long would it take to save £67,000 in energy costs?