The answer may lie in the website Obiwan posted...
the Union Flag was retained in miniature between the circles and the wing tips. This miniaturised Union Flag was also painted on the rudder.
then...
The Union Flag on the rudder was replaced by red, white and blue stripes in May 1915 and in June of the same year, the Roundel, or 'Target' as it was now known, was painted on the top surface of the upper wings. The Union Flag on the aircraft was abolished altogether.
Perhaps the flashes as we know them today evolved from the red, white and blue stripes??