I’m not out to put you down but perhaps it’s your wording of the question which is confusing. Do you mean why didn’t she take part as in sing for the finale or why didn’t she walk out?

I think it was quite simply down to preference of act they wanted and at the time Jane McDonald was quite a big deal, new album, tv show etc. None of the acts really appeared/took part in the finale apart from Bryn and Jane. As for the walk on I don’t know why she didn’t walk out with the rest of the cast and then appeared.