Context.
The UJ in the 60s was a symbol of modernism - a symbol of the music and fashion industry and manufacturing industry; the symbol of a confident nation that projected its values out onto the world*.
The use of the UJ today is far different: its use is to underline insularity and hostility to migrants, to exclude others. Starmer knows that and he's using it as a dog whistle to the millions of creepy borderline fascists up and down this land.
*Obvioulsy not for a certain community in NI at the time.