Its going to depend on who you have booked with, if you have booked through a UK tour operator, the documents will be sent to them first and they will, hopfully in a timely fashion forward these to you. They are a reasonable size, and therefore if someone isnt in,at home, you might have to collect at the local sorting office.
I understand usually
DCL sends the docs out 4 weeks before the cruise, they must understand that a lot of people will be having the cruise at the end of the vacation (holiday).
If booking through an Americian TA the docs will go through them first, If booked directly with DCL they will send to you directly.
Eeverything should be ok, however back up alternatives could be sending the docs to a WDW hotel, or even just turn up on the day with ID and booking confirmations.
Last time booking through a UK operator they turned up about 3 weeks before the cruise.