The first clip of a cannon Spitfire is Spitfire L1007 which was the cannon armament prototype. It is the only possible candidate as the second Spitfire to be converted (P9505) was not converted until April 1940.
L1007 was converted in April/May 1939, it was weighed in June and the same month tested by Vickers. Subsequently, via Martlesham Heath and the AFDU at Northolt, the aircraft found its way to Drem for squadron trials in 1940. Drem was probably considered out of the way!
P/O Proudman with No.602 Squadron, shot down an He 111 using this aircraft on 13 January 1940. He was in action again on 22nd February, the alleged date of the film. I have a legible copy of the combat report to go with that film and will post it later.
Proudman's aircraft is identified only as 'Red 2' in the report, though reference is made to his "experimental equipment", the cannon armament. 'Red 2' must be L1007.
Cheers
Steve