I am not sure that BA can be held responsible for the recruitment policies of another company - in this case the MOD. BA have an agreed policy with the MOD that allows them a managed path from the military into BA, BA do not decide whether they are homosexual or otherwise. Indeed BA is extremely diverse in it's hiring policies and attempting to suggest that a company like BA is somehow being discriminatory against those who are homosexual is likely to be laughed out of court. BA will be able to parade hundreds of gay staff in front of the court to disprove this obviously erroneous thinking. BA bizarrely enough only care that you can fly well and have the other attributes that they consider to be essential in a BA pilot.
As a wind up this has been amusing but not really attached to reality. I think you could pretty much put money on it that if you sue BA for not hiring you they will have a watertight defence. Good luck thinking that sueing a company into hiring you is a smart thing.