--- Jim Brown said:

If the Bank of England sacrificed its vaults (Very little use for
them now, with the gold sold off) and Cannon Street St was
put underground (Plus a new tunnel under the Thames), would
a tunnel connecting Moorgate and London Bridge be feasible?
Or do the tube lines around there make it impossible?

Interesting idea, Jim... There's also the problem of the slope down
from the high-level platforms at London Bridge to below the level of the
river bed. Especially since the line also has to curve from east-west
to north-south as it drops.