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.