Just spoken to a manager at Apple, since I was getting very annoyed with being told different things!
She explained, very clearly, how it operates.
Once, under carrier, it displays 'Apple Depot' (or something on the lines of that), it means all the items have been merged and scanned in. At that time, a carrier is assigned. This all happens by 5PM - i.e. close of business.
The tracking number could come online as late as 9/10PM. This is just a delay in the system updating all the new information from the day. They use three carriers in the UK - UPS, TNT or DHL.
My goods are expected into Apple today, and in theory, should be here tomorrow. BUT...if they are not scanned today, I do not have a chance.
Very annoying because I have come home just to pick up my laptop - based on what I was told on the phone initially.
Hope this helps you guys!