So if you are one down, it does not matter whether they are FTE or 33%, the effect is the same, the flight is cancelled
Just to be pedantic BA and many airlines normal cabin crew is in excess of the minimum crewing levels for departure.
It is not unusual to depart with less cabin crew (or possibly pilots when heavy crewed) than the industrial agreement requires, providing the crew agrees.
Of course this needs good industrial relations and the crew having taken the interests of passengers and company into account.