Originally Posted by Forever in Seattle

That is too far out to be worrying about. Staff can put themselves down for multiple flights, but only have to confirm/pay a day before, so numbers that far out are meaningless. And because of that flexibility, the picture is constantly changing, even hours before the flight when staff are moving flights +/- a day, or the earlier/later flight, let alone IRROP's (and that might not be directly BA, could be another airline and they are re-booked onto your flight) and the paying passengers who are the priority come in to play.
And because of all that, flying on Staff Travel bring a new level of stress, and as A P Yo alluded to, if it is a busy route, or if things go wrong, you need backup plans. Whilst your standby flight will be cheap, having to pay for unplanned extra days hotel in somewhere like HK as the flights are full and 30 staff are all trying to get back at the same time, can get expensive fast.