Order the BT line ASAP - it'll probably take days to set up, and you can't apply for ADSL (which does take several days) until the line is active.

When we moved house, we ordered both the BT line and our Zen ADSL on the same day, even though BT said it might take a week for the line to become active. Zen where quite happy to accept the order, and when we moved (a week and a bit later), we just plugged in a modem and away it went.