Because G-ZBLK arrived early on the LHR-PIT sector, this can only be staff sickness or an aircraft fault requiring more than 3 hours to fix. Because of the location and being a 78X flight, the options for BA operationally aren't good. It was cancelled around 21:30 hrs PA time, if that's a clue (suggesting technical, it's closer to departure time, staff sickness would be known before that).