@sirfredfredgeorge
I think there's several reasons why people don't confirm them:
Firstly, if there's several positive cases around, they may see no point in it, because they're certain they're right. If, for example, my dd had a positive pcr test, then ds got a positive lateral flow, the chances are it's correct, and he'd have to isolate anyway (as would the rest of the family) so I can see someone thinking they won't bother.
Then you have people that won't realise. Yes, it says so on the text, but, from experience, I can, at work, put up ten large signs that say "X that way" and half the people will still ask for directions.
Also the text says "a different follow up test." I wonder how many people assume that means to take another lateral flow test? Especially if you have several spare. I think if I got a positive test out of the blue, I might well be tempted to do a quick second one on the basis 2 false positives would be less likely. (yes I would still do the pcr ones anyway, but a lot of people wouldn't)
Then you have linking them. There must be some cases, where that fails. I know one time I got dd and ds' NHS numbers switched when I was entering them. Not sure how that would effect it.
I think you'll also have some people, not many perhaps, that would rather isolate for the duration rather than do another test. Maybe they can then work at home, and rather like it, or aren't going anywhere anyway.
If they said you can come out of isolation if you get a negative pcr, then that would encourage more people. But I also wonder whether you would then get people doing things to try and get a negative result-swabbing someone else, or deliberately trying to do a poor test.
Although on that basis, I suspect people happy to do that would probably just lie about the lateral flow result anyway, so maybe it wouldn't effect results.