Earlier quoted context omitted.
The population density in London is an order of magnitude higher than in Denver. That has an obvious impact on air congestion, which is one of the primary limiting factors on cellular network performance.
That sounded high, so I checked: London Density 14,670/sq mi (5,666/km2) ( https://en.wikipedia.org/wiki/London ) Denver Density 4,519.94/sq mi (1,745.15/km2) ( https://en.wikipedia.org/wiki/Denver ) London has a population density 3.25x that of Denver. Why exaggerate when the facts are on your side? Or more specifically, why make an explicit numeric claim without confirming that it's correct?
I don't think it's particularly useful to attempt to pin down a fixed definition of the boundaries of either city, particularly one as amorphous as London. Suffice it to say that London is densely populated, Inner London is even more densely populated and rush hour on the tube is genuinely insane; Londoners regularly encounter densities that simply don't occur in most US cities outside of a football stadium and have a subsequent need for far higher throughput than 4G can deliver.