Thanks for highlighting that Maizie.
If I weren't just an armchair observer and actually had some authority, I'd be looking at anywhere with a rate over 500/100,000 and keeping an eye. If the rate stays that high over a few days, I'd order an urgent meeting with a local public health person/local authority. I'd want to know what the local circumstances were and put in place mitigating measures.
The measures needed for each authority would probably be different, but I'd make sure the resources were there. That might be extra testing/tracing, accommodation for people to self-isolate, continuation of food parcels/shopping, financial help for those who shouldn't work, extra police to break up gatherings, environmental officers to monitor workplaces, temporary emergency restrictions ... whatever it takes.
Of course there would be people who would try to "escape" the restricted area, so people would need to be told directly that their area was in a danger zone and have it explained how important personal responsibility is via posters, local radio, social media, etc. People might develop some community spirit (a kind of Blitz spirit) to get their area out of the danger zone.
Just a thought ...