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.
"Most of the schedule" won't be good enough for some customers. They will be making their travel choices on the basis that BA's schedule could be disrupted. Some shorthaul flights will be cancelled before the strike days. This will already be costing BA money.
Bear in mind the recent 'Which' survey in which BA are slipping down the customer satisfaction tables. That's
BEFORE any industrial unrest!
It might be reasonable to ponder why????? Or who is responsible??
I imagine T&Cs on mixed fleet, might be a reflection of something much more significant for this particular company??