It's the 8th slide of this set:
assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/888353/2020-05-28_COVID-19_Press_Conference_Slides.pdf
and to be honest I have understated it, because
a) it's given as 54,000 estimated new cases over the last week, and 7x7000 is only 49,000. 95% confidence interval = 34,000 to 86,000 in the last week, so between 5,000 and 12,000 per day and
b) it doesn't include any cases in hospitals, care homes or other institutions
The 2,000 figure is just about tested and confirmed figures, and the government themselves state that the actual numbers are much higher. What matters for transmission risk within schools and their communities is the actual number of cases out there (c.0.25%, or 1 in 400 of the population outside hospitals, care homes and institutions), not the number of those who have come forward and been diagnosed with positive tests.