I think the fundamental issue here is that there's very little enforcement- just rules. So when Margaret gets covid and track and trace call her, she tells them she's been to the supermarket, to get a chippy, and to pick up some glitter at hobbycraft. The records of where people who test positive have been show supermarket, take away, click and collect.
Margaret doesn't mention that's she went round to Jean's on Monday, for a dog walk with Annie on Tuesday which they drove to in the car, or that she saw her daughter and grandkids on Thursday.
Repeat this around the country and you have some really duff data.
I'm not saying I fancy a police state btw.