If crew go sick and there are none in the same port with sufficient rest to be turned round, then it would be illegal to operate with passengers.
It's not the first time this sort of thing has happened, it won't be the last, and it happens to other airlines, not just BA. It's not good for
PR but (given teh recent incident) if it had not been BA38 ex-PEK then we'd not hear a thing about it. It is
literally an every-day event.
So you can put the conspiracy-theory books away, people.