Speaking of London, talking to building owners is exactly what Hyperoptic did to build out a fibre network in London.

Unfortunately, the building owner where I used to live wasn't interested -- it turned out most of the building was empty, people keeping apartments for occasional visits or as an investment :-(

https://www.hyperoptic.com/how-hyperoptic-works/ or https://www.hyperoptic.com/map/

Pilot is doing this cheaper than others, look into how they get their fiber in the ground...

That's what you outsource to the latest incarnation of Metromedia Fiber Network - Zayo.

Don't you have ducts? How about sewers?

The city's network of utilities long predates any sort of central authority managing them. Each utility maintains its own network of infrastructure, independent of any other; most of them are not mapped, and where mapping has been done it's not shared with the municipalities or other utilities, let alone publicly available. There are no ducts set up for general usage.

And the sewers a) aren't human-travelable like the old ones in Paris or parts of London; and b) overflow in heavy rains. You probably don't want to run electrical wiring through them.