Well, what should BA do in your opinion: to cancel the flight or what? If it is a charter airline who face equipment shortage then no problem to subcharter anyone else, because most of the charter aircraft are similar in terms of config.
However it is next to impossible to source a long haul aircraft in top airline config on a short notice. If you lucky then you will get some old 767 in 2 class config of 80s origin (so you would have claims anyway). Stop dreaming on subchartering 3-class aircraft with modern cabin equipment. They exist only at major airlines and used for their own operations (surprise?).