If you're not sure which ISP you want to use, like TalkTalk they need to install their equipment on the BT line that goes to your property.
I used to be with Oneltel and changed to TalkTalk for the better deal. TalkTalk handled the changeover with BT themselves and I didn't need to do anything. However I did contact BT afterwards just to make sure that there were no further charges owing and that it was safe to cancel their direct debit.
No, BT do not have to set up a new line every time a new tenant moves in. If the previous tenant is still in the general area, there's nothing to stop their taking their number with them; in which case they (BT) would need to reconnect you with a new number. All you need is a BT line with no broadband, just a straightforward phone line.
Once that is connected, then you can choose your provider and they will tell you what you need to do. You could of course ask BT to provide the broadband service along with the landline straight away, but that would entail your signing a contract for a year or so. If you wanted to change to another provider before that contract time is up, then BT could quite legally charge you the full amount of the total contract. It's really your choice of how long you're prepared to wait for your broadband service. Going with BT at once would be quickest, but not necessarily the cheapest. I hope this helps you to choose.