It would disappear on the basis that a DB scheme wouldn't be in the best interests of whoever took it over and if I understand correctly, and it's possible I haven't, (in which case ignore this whole post) that a new employer only has to undertake to maintain the terms and conditions you were working under before.
I think you're getting mixed up with the laws of TUPE. A new owner of 49% of NATS' shares is not a new employer, it's the same employer with a new shareholder. Every time IAG shares are bought and sold on the stock exchange the staff don't have to worry about a new employer.
A shareholder with that big a holding could put pressure on to the board to make any number of changes that might improve their return on investment, but they would have to know what they were getting themselves into when they first bought. That also assumes that one investor would buy the 49% as a whole, which I think is unlikely.
Finally, nobody 'takes over' the pension scheme. The responsibility for it rests with the trustee. The money in the pension scheme cannot be removed by anyone for any other purpose and hence it cannot 'disappear'. If a new shareholder did not demonstrate as much support for the scheme in the future then it could result in some difficult conversations between NATS and the trustee about making good any deficit that might exist.