The building, erected by the General Post Office in 1956, is plain but carefully proportioned and adorned with this nice figure of a nude telephone engineer winding cables round the Earth - those are electromechanical relays above his head.
The exchange has a certain notoriety among Cold War conspiracy theorists as one end of a secret nuke-proof tunnel carrying government phone lines. It links up exchanges all the way to Shepherd's Bush.
Now the building is shabby and ill-used. The entrance is covered with grilles and metal doors, the windows are covered with stick-on plastic sheeting and it looks as if BT keeps the place on mainly as a place to park its vans.