[frobozz]

If you look at the actual HMRC guidance, rather than some random website, you'd see that

"alterations due to advancements in technology are generally treated as an allowable repair rather than an improvement, if the functionality and character of the asset is broadly the same. For example, when single glazing is replaced with double glazing"

So a more efficient heating system, better windows, better external doors, replacing loft insulation with material with a higher R-value are all within the scope of "allowable repair".