The test before you fly back isn't necessarily a PCR test - it can be a lateral flow test, provided that it is properly supervised / certified (i.e. you can't just take out an NHS LFT from the UK and then say that you've applied it and it came out clean).To answer the substantive part of your question - it's all a matter of risk reduction since no test is perfect. The test you take to get onto the plane has to be taken up to 72 hours before getting on the plane. So this pass will filter out people who are obviously ill and reduce the risk of people getting on the plane with the virus. But it won't eliminate the risk entirely - people could get infected between taking the test and getting on the plane - or on the plane itself. And of course, there will be false negatives for the test to get on the plane. So the tests when you land are to catch those instances. If you've caught it on the plane or just before getting on to it, then your viral load may be insufficient when you take the Day 2 test for it come up positive - hence the (optional) Day 5 test and Day 8 test. So by the time you've gone through all those sieves, there should be no-one who's positive who isn't detected.Regarding the testing of people who are double vaccinated - I suspect that there are two reasons. One is that it is still uncertain how likely a doubly vaccinated person is to carry and transmit the virus - although the risk does appear to be low. The second is that while the UK authorities are happy about the evidence provided by the NHS App, it may be that they are uncertain about the evidence provided by travellers vaccinated elsewhere who are coming (not returning) to the UK
Andrzej Kowalski ● 1958d