I'm just curious if there is any information out there about why certain parts of the UK have higher positive numbers than others.
Take London and N.Ireland as examples. During the first lockdown, London was hit harder than the rest of the UK with COVID deaths and N.Ireland was seen as having fared better than most of the UK (taking population size into account). Now, all these months later, London seems to be doing a lot better and N.Ireland has some of the worst positive numbers in the UK. Is this likely to be due to London building up more herd immunity during March-May and N.Ireland are now paying the price for a lack of exposure to the virus during those months?

Yes, this has puzzled me. The PCR tests have a terrible false positive rate currently about 95% due to poor specificity. For the whole of July-August, the cases/tests rate was 0.5%. They seemed to have good specificity of at least 99.5%.

Since the rate of testing tripled around 6th September, the specificity seems to have dropped and the false positives increased. I have the impression that some cross-contamination (a major cause of false positives) has crept in due to the pressure to speed up processing.

One line of investigation I was going to pursue is where the labs are that perform these tests. If there is a geographical 'catchment area', then maybe that could be part of the explanation for increased positives in some areas.

Has anyone else looked at this?