The way it works for cabin crew is that reps are given a normal roster and all scheduled meetings have to be applied for in advance of this. BA were always very good on this point. Sometimes meetings come up post roster publication and, again, BA would usually deroster for these (including purely on TU business).
What was never acceptable for TU reps to do was take themselves of of a rostered trip. It did sometimes occur but BA would chastise the rep. In times of Industrial strife BA always get more restrictive and in the case of DH it should be noted what time of year this happened. Christmas for reps would always be easy to get time off, however if you were threatening strikes over Christmas period this facility would be withdrawn and the reps would keep their roster or get standby instead.
I would guess BA will have a watertight case on this and will be able to quote chapter and verse on the TU deroster facilities if it comes to a IT.