They are talking about the survey data, which doesn't rely upon the normal testing regime. They randomly contact and test individuals irrespective of their symptoms status, which is different from the normal testing regime that gets reported daily. I believe its a 20k sample size. The survey takes a lot longer time compile

Most recent summary: https://www.ons.gov.uk/peoplepopulationandcommunity/healthan...

Most recent data: https://www.ons.gov.uk/peoplepopulationandcommunity/healthan...

Methods: https://www.ons.gov.uk/peoplepopulationandcommunity/healthan...