London underground is prime real estate - to find a level and avoid

* the recent new massive and extensive sewer tunnels,

* the secret basement extensions of the ultra rich, multi story archival storage, vaults, etc,

* the new underground tunnels (rail / subway for US readers),

* old roman and other buried but still 'conserved' architecture,

* crypts, graves, plague pits,

* WWII UXB's etc. etc

is a hell of a 3D needle to thread - there's > 2,000 years of urban layering in that small area.

And unmapped (and changing, inadvertently diverted) rivers

There’s Probably simply too much city in the way on higher levels.

If you wanna knock out the grid, hit the substations and power plants.