I have no knowledge of the intricacies of employment law in the airline industry but, like JdS, as an employer I am aware there different rules of evidence and onus of proof between employment tribunals and the courts with the former being more pro-employee and the latter being more pro-employer.
Just because two tribunals have decided against BA I suspect that BA has driven the appeal to the high court to actually get a legal ruling on the matter which may be more in its favour.
Don't get me started on tribunals where I've been found guilty of constructive unfair dismissal against a former employee who was openly stealing company tools and property in front of my nose! I didn't actually dismiss the guy but attempted to operate within the law and clip his wings a little to stop the thieving. Alas, apparently I did not follow the correct procedures - the fact that the individual was a brazen thief was of no interest whatsoever to the liberally-minded, do-goody, tribunal. What made it worse was, as a small employer, I had to pay for legal representation whereas this individual had an entire slew of government-funded resources advising on his rights and what he was entitled to that didn't cost him one pretty penny.
Now he has our tools, our stock, six month's pay and it cost him nothing whatsoever. I couldn't dream-up more efficient employment laws than we currently have to positively encourage employees to behave in this manner!