That's most certainly not cable. It looks like a robust tube. Perhaps it's something fibre could get blown through. Any clue through soil/pavement disturbance where it might run too?
*** update ***
I don't want to get your hopes up (are you in an intervention area), but a bit of research reveals there is something called "direct bury sub-duct", which is just a fancy name for a plastic tube you can bury in the ground for carrying small fibre bundles. I've no idea if BT use such things, but there's some in this brochure from a telecoms ducting supplier available in a range of sizes.
(Using the term "sub-duct" for a single tube buried in the ground is a bit odd as such a thing is surely just a very small duct).
nb. Radius Telecoms are a BT supplier, but I'm not suggesting this is necessarily one of their direct bury sub-ducts.
Edited by TheEulerID (Mon 14-Sep-15 19:30:37)