TUGNBAR - there are 2 issues. First, does someone (other than BAA) want to own STN, and second, how much will they pay ?
Suppose BAA were willing to sell for £1m - highly likely that some company will want to pay for STN. Now suppose, BAA want £10bn - almost certain nobody will pay up.
I don't know what kind of bidding / auction mechanism will be put in place for a possible sale of STN, but it seems most likely that at least one bidder will emerge if BAA put up a "for sale" sign