I would like to note that discrimination works both ways on any particular characteristic. Just because the common cases of age discrimination are old people, of gender discrimination are women, of sexual orientation discrimination are homosexuals, of ethnic/racial discrimination are people from Black, Middle-Eastern and Asian backgrounds, does not mean that discrimination the other way is any more permitted by law.
Having hiring or firing criteria that penalise younger more than older, or having all the gay managers fire the straight guy because he's not gay like them, a company run by BAME people refusing to hire anyone White, and so on, does not make those ethically correct or legal either.
Not only does LIFO risk age discrimination, it also risks discrimination against other characteristics such as ethnicity or sexual orientation - because those employed longer were hired when discrimination was acceptable or not enforced, a company may well have hired only straight white middle class British people in the past. Having more recently provided opportunities for all, as legally and ethically required, they might now have a more balanced workforce but all the BAME, gay, etc, employees were hired relatively recently. This is particularly important for "lifetime career" companies, and BA is one of those (railways are another). If BA makes the most recently employed people redundant they'll lose many of their non-White, non-British, non-middle-class employees. That would also be discrimination, even if not intentional. BA has a good reputation for treating people much the same regardless of origin, orientation, etc (they all get treated equally badly these days...) but even inadvertent discrimination is still discrimination.
An employer must act positively to avoid discrimination in this day and age, mere passive uncaring is not enough. Lawyers will come calling if they do not.