Regarding the photo, looking closely at the line on the cable it looks like fig8 cable, like the (usually black) overhead cable.
T&E at first glance, though. The 'line' caught my attention when I zoomed in.
Looking closely it is a fig 8 cable and is about 30m underground to a BT post in a conduit that is cemented over. Hope the SFI engineer is good at blowing cable if he is going to replace it.
It was put in in the 80s during BT's free re-connection promotion.