As efficiant as SERCO are, in the current economic climate,
1 - they do not have the cash, and
2 - they could not raise the cash to buy out TAG and then raise the extra money required to keep NATS running.
Aviation is a
very small part of the SERCo groups' operations, and they are unable to, and always will be unable to, run an organisation like NATS, for the same reasons which applied when their first bid failed.
End of story.