I'm confused now!
If it's a BT line (even if the rental was paid to Talk Talk, Post Office, Plusnet etc), it could be fed with a dropwire from a pole, or direct underground.
But, if it's cable, it can only be Virgin, unless maybe it's an old NTL or Telewest line that was ceased before the three companies merged into Virgin, in which case perhaps Virgin would now have no record of it. Whatever, it would be directly underground fed.
So, do you have a dropwire, and if so can you trace the wiring to this BT socket?