A problem with using a diagnostic test for mass screening of an asymptomatic population is that you may end up with 50% or more of the positive results might be false positives. Has to do with the Bayes Theorem, which I’ve detailed elsewhere on the blog.
Good news – if you get a positive test, you can rule out many false positive results just by doing a second test on a second specimen sample. Unfortunately, some programs don’t do that – any positive result, even without symptoms, means 10-14 days of quarantine, often for the entire family
I asked some people online about the UK program – there the plan is that anyone getting a positive rapid test result will then be tested by a lab using PCR test to confirm or rule out the first result. That’s how it should be done.