The numbers aren't that low. For each confirmed case, there's a much greater number of infected people who haven't been tested.
Important to highlight test positivity rate, but, for the purposes of this discussion, it's also important to disaggregate that data by location, eg. cities or regions currently seeing a surge in community transmission. The problem with not testing the majority of people who are infected remains. The defense of the test & trace programme is not persuasive; a well-functioning test-trace-isolate scheme aimed at curbing community transmission is vital to protecting the people involved in those "complex cases". Adequate surveillance of people in care homes etc. is the bare minimum, and comes very close to shutting the stable door after the horse has bolted; it should not count towards the NHS T&T's points.