If it's even possible, and if they have a means of even ordering such a thing, then FTTPoD or FTTP might be the key?
In fact, what if you were to just forget about
moving, and just try ordering a second line but ask about the route, with the assistance of a good ISP (
) who could fight with BT, you could get the job done, and then cease the old line.
Of course, if you could get FTTP, who cares about length.