Plenty of availability still during the summer, so BA certainly have not 'sold out'!
Buster no one said they had 'Sold out' all season. The article is about flights being 'sold out' this weekend.
Regardless of sold out or not, it's about yield. You can fill a plane to the brim, but if those customers are paying peanuts it won't last long. Having seen some of the BA fares mind, they're not cheap now that pretty much all the low fares have sold. But you pay for what you get in this day and age. It's nice that Stansted passengers are now getting a choice of low cost or full(ish) service from their airport.