That is going to be very quick to produce. Dads Army took a lot longer to bring to the screen.
Also the story at the time were no RAF fighters over the beaches. What fighters there were were Hurricanes and Spitfires mainly fighting in land to try and keep the Jerry off the beaches. Dowding wouldn't release more sqns knowing the next battle would be for Britain.
I suppose the Spit/109 action is over 'Dunkirk'.