As I posited upthread, it should be possible to get away with a short time period of high frequency testing. After an initial burst of say 2-3 weeks, we'd essentially be in a near-elimination position and the only testing would need to be for those returning into the country and those exhibiting symptoms. Sustained high frequency testing of the entire population is pointless as it means that either the test itself doesn't pick up on all cases (at which point, why bother continuing to use them) or the test and trace system is broken (and would be the better return on investment to fix)
That might work if the test was 100% accurate. But no test is 100% accurate and the rapid tests thus far seem to be less accurate than the swab tests. Even though they are highly accurate, the swab tests are producing perhaps 400 false positives a day (ie a third of the reported cases are false positives at present).
If you scaled the swab tests up to 10,000,000 a day then you could expect something like 20,000 false positives per day. Rapid tests would likely have a false positive rate an order of magnitude higher. So 200,000 "positive" test results every day (1,400,000 per week) from people who don't actually have the virus. How do you manage that?
Even if the spit test was as accurate as the swab test, that's still 140,00 false positives a week.
Similarly, a small proportion of people who do actually have the virus will falsely test negative. This isn't such an issue whilst there is very low incidence in the general population, but if numbers did pick up again then you would have people with a "clean" certificate who are actually infectious.
This is why the government chucked all of the rapid test kits they bought in the spring in a skip. They just weren't accurate enough to be useful.
So wide scale testing to provide an individual "passport" is scientifically flawed - one of the reasons why the WHO do not recommend such approaches. Testing should be used only to confirm suspected cases, people found through contact tracing and to monitor incidence in the population.
As I said upthread - I would actually support such an action for a short period of time if it meant that we could ditch face masks and all of the other ridiculous measures that have been put in place and get back to normal. But I'm under no illusion that it would in itself be a nonsense measure - at least it wouldn't be so intrusive on our lives and would quickly come to an end when either the Facebook Karens finally realised the sky hadn't fallen in and/or when Matt Hancock realised it's completely fruitless to chase your tail in this way.