The METAR for BOH on the morning of 13/4 shows a minimum visibility of 6000 meters but that was at 0520.
The visibility for the arrival time of the GLA flight was 9999 and cloud base BKN026.
Even if we presume worst case scenario of crew and aircraft being only CAT I there was no weather related reason for cancelling the flight.
Being so obviously well informed CC perhaps you'd like to tell us why it was really cancelled?