A United B767 can stand at LHR for 5 hrs, and then operate its flight. No canx.
It doesn't come back.
A BA A319 will stand there for 5 hrs, and then operate its flight, but arrive back in LHR about 8 hrs late. BA runs out of short haul aircraft.
Here in Stockholm we operated 3 out of 5 flights to LHR. First on time, second 5 hrs late and third 5 hrs late, both waiting for landing slots at LHR. The third left ARN at about the time the 5th should have.
There was not enough runway capacity at LHR.