Factual error: When George VI and Churchill are having lunch at Buckingham Palace there are servants in the room. In reality, servants were not present at their wartime lunches due to the often highly secret content of their conversations. Food and drinks were left in the room by Palace staff and the King and Churchill served themselves.
Factual error: As they are leaving the war rooms a car can be seen parking in the background. This car is an Austin A30 and these were not introduced until the mid 50's.
Answer: It was probably Perspex, an acrylic plastic commonly available at the time, used for, amongst other things, fighter plane cockpit canopies and windscreens.
stiiggy