The issue I have with testing (happy to be corrected)...if you have no reason to suspect you have the virus said person will not get tested, Once symptoms develop they may get a test and be told yes you have it, which by then you will have infected countless numbers of people, If you have symptoms its wise to assume you have the virus and isolate, my point is once you have symptoms is there really a need for a test
Add to this that you could be tested at 9am and be negative and tested again at 3pm and be positive, I suspect that is one of many reasons why they are not bothering with airport testing in the UK right now