The equivalent of the entire population of Glasgow passes through Kings Cross station in the morning rush hour. There are 4 other stations with similar flows. Over 20 million people live within an hour or 30 miles of Piccadilly Circus.

That level of population flow and population density are not as special as you are making them out to be. The transfer station nearest Seoul's most recent cluster (discovered 7 days ago) gets about 25% more annual traffic than King's Cross, according to Wikipedia (125,000 per day at Sindorim Station vs. 34 million at King's Cross annually). Despite the cluster having developed at the beginning of March, Seoul is finding fewer than 100 new patients per day. With masks, reduction in ridership, cancellation of meetings, extensive WFH, extensive testing and isolation measures, this epidemic is not as hopeless as the UK seems to be making it out to be.

I completely agree. I was shocked to hear that only people that present with symptoms that require hospital treatment are now being tested.

There are 5 other international airports that call themselves "London", not just Heathrow. And of course London also has road and train connections.

I was mainly thinking about road and rail traffic into and out of the city, including daily commuters.