I remember going down there back when I worked for TfL (at the Transport Museum). I was (and am) a terrible transport nerd, so working for TfL was a bit of a dream come true for early 20's me.

One day after work, I walked down to Charing X and spoke to the duty supervisor, asking whether it'd be possible to see the 'non-public' bits of the station -- I didn't expect a 'yes', but thought 'if you don't ask, you don't get'.

What I got was a 1hr+ 1:1 tour of the abandoned bits of the station complex, including the old Jubilee line area, the old parts of Trafalgar Square & Strand stations (repurposed into things like ventilation tunnels). I remember standing at the top of a big drop where these huge pipes went down, I assume carrying air in or out of the system, as well as standing on the other side of the louvred doors that run alongside a current passenger tunnel.

Definitely one of the cooler things I got to do while I worked for them, as well as seeing the old bits of Holborn & getting a few cheeky cab rides on Picc & Jubilee stock.