Kingspan K118 is a "high-performance insulated plasterboard with a thermal conductivity of 0.018 W/mK. It is suitable for internal wall insulation, dry lining and quick response heating".The Kingspan Kooltherm K118 insulated plasterboard has a fibre-free rigid thermoset phenolic core. It is suitable for mechanically fixed or plaster dab and adhesive bonded dry-lining. With a 12.5mm plasterboard bonded to its front surface, it has a composite foil based facing on its reverse surface. 

I have four full sheets (plus some offcuts) of Kingspan K118 37.5mm insulated plasterboard left over from a retrofitting project. It's good material to work with and offers very effective thin internal wall insulation. The purchase price was £80/sheet. For the avoidance of doubt, I'm selling the four sheets together. Available now - collection only - from Kirkcudbright in SW Scotland.