There is this message from PHE. you find it in this wiki:

https://en.wikipedia.org/wiki/COVID-19_pandemic_in_the_United_Kingdom
On 3 October, Public Health England announced that a 'technical error' had caused under-reporting of new cases for recent dates, and that the missing positive results would be declared over the forthcoming days. The number of new cases declared on 3 October was approximately double the rate prevailing over the preceding few days.

If you find a set of -ve and +ve results that you left out of your data, and 'fixed it' by only declaring the missing positive results as PHE said they would, the effect would be to alter the ratio of -ve to +ve recotds in the data, so around Oct 3, you would have a relative surplus of +ve results, giving the impression of increased positivity, as you snapshot shows. It looks like the data they had was not timestamped, so excess records got clumped on Oct 3.

It is deeply shameful if data has been so abused by PHE and others. They have no idea how to do things properly. If you have a data set, and some section has gone missing, it can only be repaired by accounting for all the records, not just the +ve ones. PHE is staffed by cretins so it appears.
My advice, drop records for a few days either side of Oct 3, they cannot be trust for data trends.

SQL and databases exist for a reason.

True, it is possible to treat data rigorously irrespective of the record storage system. We used to use paper tape, and it worked! Users of desktop applications are not famous for treating data rigorously 😀

A rolling seven day average takes the noise out of the figures. Well worth doing to help you see what's really happening.

The positivity trend in Scotland has a distinct correlation with weekends. During weekdays test numbers are high but at weekends they drop by 4 or 5 thousand, and the positivity increases by several percent. It is very predictable. I queried this with Public Health Scotland but they could offer no explanation other than less testing of key workers, care homes, etc., at weekends. It suggests either lots of infected people turn up at weekends for testing or the key workers, etc., that are tested during the week are invariably negative i.e. less uninfected people get tested at weekends. Perhaps we should only be testing at weekends to get a better picture of prevalence.

The clear correlation in Wales was secondary schools specifically.

Absolutely nothing else fits the data.

All of this data is noisy though, as above the only sensible way of dealing is by specimen date (or date of death) combined with rolling 7 day averages.

Im assuming the UK does absolutely no quality assurance by comparing different lab percentages and dummy samples etc. It seems we have no interest in monitoring labs for consistent output.
Maybe thats worth a FOIA request.