1st off, the Sky installer should not have touched the old NTL wires.
The fibre optic cables run to the large green cabinets, from there to the house and the modem, the cable is coaxial (better than satellite quality aerial cable).
Unlike ADSL which relies on twisted pair cable and your proximity to the exchange, with Virgins Network the green boxes are never that far away, so speeds are pretty reliable (I get full connection speed 24/7).
Hope that helps.