The version of the Union Jack approximating the 1800 Order in Council draft approved 5th November by George III (with shortened aspect ratio compared with the official 5:3 (land) and 2:1 (sea) proportions in use today, but not quite square). See the original draft illustration from the Order in Council.
Flag_of_the_United_Kingdom_(3-5).svg
Flag_of_the_United_Kingdom_(3-5).svg: Original code by Stefan-Xp with modifications to ratio by Yaddah.
derivative work: NikNaks talk - gallery - wikipedia