Many of
Dark Blue World's air-to-air scenes were taken from the extended Battle of Britain film (originally out takes). That's why the new air-to-air footage had to have the same squadron codes to make it all fit in. I have an article about it somewhere in one of my aircraft magazines, but they're back in the UK!

gofaster:
Angels One FivePLOT DESCRIPTION
"Angels One Five" is the cognomen bestowed upon a group of WW II British fighter pilots. The squadron leader is Tiger Small (Jack Hawkins), who is taken out of commission after an accident. Despite the protests from his fellow flyboys, Tiger insists upon taking to the air again, thereby setting the stage for the film's exciting and inspirational finale. Angels One Five differs from other combat films in that the battles generally take place offscreen; the progress of the principal characters is relayed to the audience via radio reports and control-room charts. If this sounds dull and static, it isn't: in fact, Angels One Five is among the best of the "Battle of Britain" war epics. ~ Hal Erickson, All Movie Guide