I lived on the isle of dogs in docklands up until around a year and a half ago - Probably one of the densest populated areas for fibre optic cabling to supply canary wharf etc (plus the communications companies based there).
However the isle of dogs was a considerable distance from the Poplar BT Exchange so DSL was limited to 1.5mbps and the cable network around the Isle of Dogs is so old that 1) it is still analogue, 2) Virgin were looking to just switch it off as spares were no longer made and 3) Virgin had no plans to upgrade the network as did not have the money or the customer base to support it (everyone had moved over to Sky as cheaper, more channels (only around 30 on cable) and better picture quality).
Whilst the cables may be able to carry a digital signal, the investment in cabinets and head end equipment preclude this where the investment cannot be recovered.