The DC pension is underfunded because if the contribution rate cannot possibly produce a reasonable fund to purchase a pension, it is by definition underfunded.
To you it may not be adequate, but by the standards that an 'underfunded' pension scheme exists in both the media and accounting terms, it is not. The company owes no further money other than what it gave to BAPS in contributions for members. The NAPS and APS old DB schemes however require the company to make up the shortfall - at least in NAPS which is underfunded - over the next 10 years or face remediation action from the PPF to prevent the state being overloaded with promises made in the past that companies cannot fulfil.
BA have made no promises on what you shall get from BAPS - capitalism dictates that when the terms are insufficient that BA will not be able to hire the staff.