I'm sure there is a black market, but it is a bit more complex for most cable. By the time it has been stolen, it is more or less useless as cable (and I don't think Network Rail are desperate enough to be be buying it back on the dark side, so who else would it be sold to?). Instead it has to be recycled as metal scrap, which means machinery to process it (removing insulation) and ultimately a route back into the supply chain. Or maybe (like everything else we don't want, and quite a few stolen cars) it gets exported now? If it is being processed in the UK, it should be reasonably hard to hide a factory of any any scale (with associated workforce, buildings, energy supply ...) - assuming the resources exist to go looking for it.