It is not just a fibre cable
BT state :-
"A large thrust borer being used by a third party construction contractor on work not related to BT‚s business penetrated one of our deep level tunnels in that area. The thrust borer has completely penetrated the tunnel and has caused material damage to both the copper and fibre cables in the tunnel.
BT engineers are working around the clock to restore service as soon as possible.
More than 70,000 PSTN customers were initially affected, though almost half of these have now been restored. Several thousand broadband lines, a large number of private circuits and Featurenet services have been impacted. Emergency and Police services were also affected but these services have been restored.
The tunnel is so badly damaged that it is not possible to work there pending structural support work and it is likely to be three days or more before that work can be completed and damage repair work undertaken. "