Obviously BA and BAA would like the extra runway and mixed-mode.
If they get the runway, it would be easier to introduce mixed-mode at a later date, rather than the other way round. The conditions set out by Hoon regarding a cap on extra aircraft movements and no to mixed-mode, can be easily changed.
In fact, according to the studies NATS have done, the optimum configuration of the three runways for maximising movements and avoiding ground congestion is to retain the two existing runways on segregated and operate the 3rd on mixed.
I don't think Hoon specified anything about the configuration of the 3rd runway. Obviously operating it on segregated along with the other two would be ineffective as either take-offs or landings would still be confined to one runway.