MF earn more than £12k. That's the basic salary and then they are paid an hourly rate for the duration of their trip and receive bonuses for attendance etc.
Soundings are that a lot of the union concerns are more to do with the rostering and rest periods rather than pay itself.
It seems that calling a strike on Christmas Day and Boxing Day is more to do with putting
PR pressure on BA rather than causing disruption. A very reduced schedule operates on Christmas Day in any event and BA seem pretty confident they can cover most of the schedule, including all long-haul flights.