Hi loulee. I think the passport office now says that they don't process same day passports anymore, except in extreme situations. Their turnaround time is super-quick now, so if I were you I would send all the relevant documents today by registered post. They probably put it as a high priority when they see your planned date of travel.
If you're really concerned, phone the passport office first and have a chat. They are human beings so they'll give you sound advice!
You'd be amazed at how willing they are to help in last minute applications. I'm always applying for visas with my job and most of them are done 2 days before travel!! Always fun 'n games here...