The article states that it is not known where in the country was affected, but I think we can deduce that. Here are the daily cases by specimen date. Dotted line is Saturday's data, solid line is Sunday's. The difference between the two is partly the normal daily update, but around 2/3 of it is correcting the underreporting:
Looks like all regions were affected, but by far the largest corrections are in NW, NE and Yorkshire regions. In particular, NE had looked like cases were declining, but we can now see this was incorrect, and they're still increasing rapidly
Edit: note the most recent 3 days are always incomplete, so any decline shown there is not a real effect.
Nit: presumably whatever the maximum time is before results is the period in which updates to past results should be expected. I think it's about 1 week at present?
Before the recent leap in cases my test took 6 days to come back (negative). My wife's test at the same time came back the next day. At the time they were saying that 72 hours was the expected return time for results. For me it has been a couple of days of steadily worsening coughing, and I gather people take about 3days-1week to show symptoms ordinarily.
So UK results are most likely reflecting infections from 1-2 weeks ago.
7-day moving average: http://danger.handley.org.uk/misc/rates-uk-recent.png
Raw data: http://danger.handley.org.uk/misc/rates-uk.png
Looks like all regions were affected, but by far the largest corrections are in NW, NE and Yorkshire regions. In particular, NE had looked like cases were declining, but we can now see this was incorrect, and they're still increasing rapidly
Edit: note the most recent 3 days are always incomplete, so any decline shown there is not a real effect.