They have stayed above the market rate, because they have a strong union. BA made increasing profits, year on year, until the world went mad, whilst paying crew above market rate. Why shouldn't employees get a slice of it and why would a union actively seek to reduce terms and conditions of its members. £B1 or £B1.2 does not make the slightest bit of difference to the employee, but taking 25% out of terms and conditions does.
The reason that BA crew can maintain their package is because BA is what it is, THE major stake holder at LHR it was built as a nationalised beast and gained its market position because of it as it grew as a private company. What a great place to be at the right time, as many senior BA crew members were.