Vass,
Adams Foam board (AKA, Dollar Tree foam board) is 5mm with paper backing on. Paper is pretty thick and heavy, so it'll loose about 1/2 mm and half it's weight when stripped.
Dunno about a UK supplier, but if you ever drop by Georgia (the US one, not the one south of Russia), there's one in almost every town!
BTW, Welcome to the forum! feel free to drop by the flight scool (recently moved into "lessons learned") and "Say Hi!"