When I ported my number BT number to Virgin, it was up and running before he left.
All he done when he'd installed it was ring up someone and register the line, it took about 20mins total.
One thing though there are a very few engineers who might try and hook your new line up to the existing BT connection, as this saves a lot of work on their part, don't let him its illegal. Before he starts say you want your VM connection put next to your old BT connection, or wherever else you want it, he'll then know you know what your talking about. As I said though its not very likely, BB and telephone should be done within the hour, don't forget to let us know how it went.