Darkest Hour: In the 'War Room' scene, there appeared to be a sheet?
I havent seen the film. Assuming its solid, not flexible, it would most probably be Perspex. Which was readily available, and commonly used to cover wall maps and table maps. One of the advantages of Perspex is that if you light it from the edge, then draw on it with coloured wax crayons, the crayon glows. So it is excellent for overlaying notes or battleplans on a map.