How to add an icon to a keyboard layout?
I am not sure if ukelele itself implements a way to do this, but the solution turned out to be trivial.
Locate your .bundle file which is being used. Mine was at /Library/Keyboard Layouts/#British.bundle.
With a terminal, cd into the Contents and then Resources subdirectories of the bundle. Now the command open . should open finder at this location.
Have fun replacing the .icns file within.