The taxable value of a benefit, whether contractual or not, is not the market value of the benefit, but what it costs the employer to supply it. If it cost BA more to supply than the employee pays, there would be a taxable amount even if it were not contractual.
As staff travel is on an otherwise empty seat, and is profitable in respect of the 90% discount tickets, no tax would apply.
I think, but am not sure, that BA already makes a 'settlement' payment agreed with the Revenue in respect of the 100% discount tickets. As they are still technically otherwise empty seats, the only cost is the supply of food and drink. Staff are specifically barred from accepting the in flight goody bag, I'd guess this is make sure the revenue don't include the value of that in their settlement calculation.
So if Unite wins the case for staff travel being contractual, it may make no difference to it being tax free.