Do BT have cables in the ground straight from the house
yes...you'll see the box on the side of the house somewhere and the line will go straight into the ground to some sort of pavement box in the street...from there it'll just carry on back to the exchange but it might go overground at some point depending on how the network was built in your area.
BT Infinity 8th July 2010
Connected to: P23 Kilmaine Road, Bangor, BT19 6DT (NIBA
600m (approx) to cabinet
25.5mbit down / 7.6mbit up
BT Broadband, roughly 4mbit sync
4KM line / 54dB atten / 9dB SNR / Netgear DG834GT