Hello Hugo, good point; I should have given some references. From what I can remember, and from my browsing history, I used for instance (but not limited to):

https://www.greenspec.co.uk/building-design/insulation-materials-thermal-properties/

https://www.greenspec.co.uk/building-design/u-value-introduction/

https://www.thegreenage.co.uk/article/thermal-conductivity-r-values-and-u-values-simplified/

and many other UK sources such as

https://www.kingspan.com/gb/en/campaigns/approved-document-l-england/

https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/1133079/Approved_Document_L__Conservation_of_fuel_and_power__Volume_1_Dwellings__2021_edition_incorporating_2023_amendments.pdf