From Wikidpeadia
"Quill actually flies a Spitfire in the film, and had tested the Spitfire in battle, shooting down three aircraft while on temporary assignment to the RAF. Jeffrey Quill's log book records that the flying sequences for the film were made by him from Northolt on 1–2 November 1941, in a Spitfire Mk II."
and
"Film footage of the Supermarine S.4 in taking off from Southampton Water, and in flight, which is now available nowhere else. The film also includes footage of many real-life Battle of Britain fighter pilots in the opening and closing scenes. RAF fighter pilots such as Tony Bartley and Brian Kingcombe (with pipe) have cameo roles in the scenes at the dispersal"
I'm guessing - but maybe the original Supermarine films were lost when the factory was wrecked by bombs?