"Line Plant' is a bit of a cover-all term, it could be anything really.
You say that the property previously had service, but then maybe when finished with, part of that pair got used to provide service or repair another customers line.
The people to chase are BT themselves.
Yes the last tenant there had service less than 3 months ago, I chase with them daily, the first excuse they gave was a narrow band fault, then yesterday it was this line plant work which has been approved by their planning department, I've had 4 different activation dates, I wouldn't mind not having fiber for a while but there is no phone service there ether which means I can't do anything at the address
even BT WIFI does not work there.....