I had an ISDN connection to my home for business use which I then had disconnected when I went over to Broadband. Like you, BT left all their stuff including the new wire that they'd strung from the nearest telephone pole.

I have disconnected the internal box and dumped it, pulled the wires back through the wall and plastered up the resultant mess. The legal position? I have no idea but I used the principle that I wasn't paying BT for a service and they'd left all their rubbish lying around in my house so they clearly didn't want it back.