That said, the UK was testing many people in the initial phases, and out of 20,000 tested, only 150 ish were positive. If the virulence was that high, we should have seen lots more of the UK tests positive and then for PHE to track those people and find no symptoms developing.
Yes, but it's believed that it had been circulating in Northern Italy for some weeks before it really took off. Almost all of our cases discovered came from those who flew back from Italy/the Alps. You wouldn't expect to find too many cases in such situations but it will really have ratcheted up since they only started testing in hospitals. Plenty of other cases would have flown in from elsewhere and begun more widespread infection. No surprise that London and the South East would be a hot spot given the packed commuter trains in that neck of the woods.
We'll just have to hope that the majority of infections will be asymptomatic/quasi-symptomatic with just a proportion showing the typical symptoms. At least, then, the number of critical cases might end up being lower than feared.