I'm imagining drones putting sensor domes with ~100 laser diodes each on top of existing street lights, using in the order of a few watts from a small solar cell and battery to do basic signal processing (most windows will be silent most of the time) and uploading anything possibly interesting over a standard mobile data connection.

Then process the possibly interesting signals with a voice-to-text system similar to Alexa/Siri/Google/Cortana and now there's a searchable text database of everything spoken next to a window in a city.

That would be, what, mid tens to mid hundreds of millions of £ for London? Compared to the annual budget of £3.24 billion?

edit: possibility->possibly