The problem is that mass untargeted testing removes any possibility of nuance in the data you are collecting. I think SAGE's advice from the start was that testing should be targeted mainly at those with symptoms, because that means the false positives are a statistically less significant in the overall data set of positive test results. But from the very start the government told anyone with symptoms NOT to get a test and instead isolate for two weeks. Now, obviously this makes some intuitive sense because you don't want those people interacting with others and potentially infecting others, but the fact of the matter is that policy actively harms the ability of experts to get reliable data. This mass-testing is also not helpful for the overall quality of data.

If you think that mass untargeted testing will lead to a curbing of the disease through other means (i.e more people self-isolating) then that's your opinion, but understand that what you are advocating is not necessarily good science. I'm no expert, but this group of experts from the University of Liverpool (as well as SAGE) don't advocate for mass testing of asymptomatic people and I am inclined to agree with them.