How much it will cost you to go green

The whole article is nonsensical, they use absolute values for something that depends on surface and always reference the EST (i.e. the Energy Savings Trust) for the data they provide without linking to the source.

The site:

https://energysavingtrust.org.uk/

seems like one of those "explanation to the masses" ones with no actual practical value.

Personally I do not even understand what they do or are supposed to do, it all seems to me like a lot of "fluff":

https://energysavingtrust.org.uk/about-us/

(i.e. are they government backed or are a sort of lobbyist for green building energy saving?)

The 640 comes from a graphic here:

https://energysavingtrust.org.uk/advice/roof-and-loft-insula...

27 cm (what?) of mineral wool, and it seems like they propose a DIY job.

Of all the possible ways this is the cheapest one (and only can be done in a specific type of house/roof/loft) and if it is unused.

In the UK, the material only should be (roughly) around 0.40 per cm of thickness per square meter, so 0.40x27= 10.7 pounds/sqm, installation is very fast, according to them you just unroll the mattresses, but there is transport anyway, so the cost is likely to be more like 15 pounds/square meter, that gives us around 42 square meters of the loft in a semi-detached house, it is plausible, but can apply to (say) 1% of houses, all the others will require professionals and much, much more money.

And the "If you want to use your loft as a living space, or it is already being used as a living space ..." is a good example of British humour, so you have a loft used as living space and you lower its ceiling 27 cm (actually 28+ because of the plasterboard) and make it narrower and shorter 28 cm on each side ...