I'm not sure you can order before the previous owner has relinquished the line. Otherwise it could lead to all sorts of issues with 3rd parties ordering services maliciously resulting in the persons line being disconnected. Until they have released the line then I believe there would be a tag on it that would stop you placing an order (I'm sure that's how it used to work).

Hi

1 month I'm having to wait for a new BT line and FTTC after a house move (it does have a BT line but disconnected), reason BT Openreach gave was lack of resources. I say do it as soon as you can after exchange of contracts, although in my case that would have still meant a 3 week wait.

Regards

Phil