In essence, a tunnel under Whitehall was built using conventional tube tunnelling techniques. Cables from here could be routed up to the then Trafalgar Square Station (now Charing Cross), run round to the Bakerloo Line and then through that tunnel down to Waterloo Station. Here, the cables routed round to the Waterloo & City line and not far from Blackfriars station, a narrow pipe was sunk to run the cables up to the Faraday Building.
Other cables were routed from Whitehall to systems outside London also by being laid through the tube tunnels and then to overground networks away from the main bombing targets.
This secret government “escape tunnel” is actually just a service tunnel – albeit rather a large one. Q-Whitehall is the (possibly unofficial) name given to it. Although not really designed for regular human use, it could be used as a route between Whitehall buildings in emergencies, such as during gas attacks.
Back onto the rumours, it is widely known that quite substantial upgrade work was carried out in the 1950s, and the file for that is locked away in the National Archives awaiting declassification. Put a note in your diaries for 2026, as that is when they will be released. Another unfounded rumour is that a further upgrade was carried out about only a few years ago – but that came from just a single, if usually reliable, source a few years ago.
Also, the Whitehall tunnel is reported to be linked to the deep level telecoms tunnels (allegedly) constructed by the Post Office during the cold war. Although British Telecom won’t talk about their network, enough reliable if anecdotal evidence exists to show that it was a very real project. Not to mention, a journalist who broke one day and took photos. https://www.ianvisits.co.uk/articles/more-on-the-secret-tunnel-under-whitehall-1760/