I've written to B&Q:

Looking at Thermawrap General Purpose Wrap on your website, it says: [Thermal resistance 1.455m²k/w in Wall Lining installation]{style="font-style: italic;"}. Since its thickness is quoted as 4mm, this would give a thermal conductivity of \~ 0.00275 W/mK. Expanded polystyrene has a thermal conductivity of 0.033 W/mk, so are you saying that Thermawrap is a \~12 times better insulator than polystyrene?

I'll let you know if it's really true!