I fail to see how this is in any way sex discrimination.
If a man under the same circumstances said he wanted to fly 50% to spend more time with his family then I presume the airline would also have said no.
I believe that 50% is available, but to pilots with sufficient experience to maintain a safe and efficient operation, and that has to be the airlines first priority.
The other crew members and passengers have a right to expect a certain standard of recency and experience, and whilst managers and others do fly fewer hours than line pilots they are usually well etablished in their careers first.
Having only obtained just over a years worth of flying in the last five years (according to some of the press reports) I think it is not unreasonable fro BA to act in the way that it has.
If it DID give special permission because she was a mum, yet rejected similar requests from dads then that WOULD be sex discrimination.