This is a modification of the file found here - https://en.wikipedia.org/wiki/British_Civil_Air_Ensign#/media/File:Civil_Air_Ensign_of_the_United_Kingdom.svg. But the colour of the field of the flag has been changed to Sky Blue from Air Force Blue, in keeping with the historical example found in a photograph here - https://en.wikipedia.org/wiki/Civil_Aviation_Authority_(United_Kingdom)#/media/File:19870830-EGSU-DH104-GALFU.jpg The colour correction fits both the historic description of the flag and contemporary examples. See here: https://www.crwflags.com/fotw/flags/gb-ciair.html
this is a modification of a file already on wikipedia - https://en.wikipedia.org/wiki/British_Civil_Air_Ensign#/media/File:Civil_Air_Ensign_of_the_United_Kingdom.svg. the original image is public domain. i am only correcting the color (of the field of the flag). the file is otherwise identical to the one indicated above.
shorepine