For all the improvements at Stansted it is worth remembering that these BA summer flights are primarily about using BA aircraft that would normally be stuck on the ground at London City airport for 24 hours due to the weekend curfew. Aircraft on the ground means they are paying airport fees but not earning any revenue whatsoever.
Anything else beyond making revenue out of normally curfewed aircraft is purely a bonus to BA.
Note to pedants - yes I know strictly it's BACF but they are 100 % owned (give or take) by BA so as far as IAG shareholders are concerned it is all BA.