I visited London this weekend and found even many underground _stations_ were without mobile service, 15 years after I got used to having spotless end-to-end mobile coverage even in deepest tunnels. I don't know why the London Underground lags that of other cities?

Knowing the next station is usually a solved problem that doesn't need a smartphone, because that's displayed in the train itself and called out on speakers. But once you are on the platform and you need to ask the route planner what the fastest route is to a specific station (It could be walking to the surface and taking a bus, so it's not as simple as looking at the subway map) - then you are out of luck if platforms don't have 4/5G coverage!