ACAS can't do anything if one or both sides will not concede anything.
In BA's case, it cannot remove imposition, because it cannot afford to (reminder: it is losing around 50 million a month) and it no longer has the staff, having released them through VR.
BASSA could afford to continue working under imposition, but refuses to.
BA is fighting for its survival. If it dies, BASSA does too.
I would think that mere fact alone would concern BASSA members enough to demand explanation for their leaders' actions. (At the very least I'd ask why Malone can prove herself to be so incompetent and still come out of it smelling of roses).
But apparently, it isn't.
I can't see anyone can mediate when one side can't move and the other one won't.
Seems to me this is the root of the problem.