Yes there's a big flaw in it but most people have a moral conscience that leads them to do the right thing. Maybe I'm naïve, but I can't see loads of people bothering to upload the code without actually doing the test. It's more hassle to go through the test questions on the gov website than the...
What it doesn't say is seriousness of illness & hospitalisations etc. When Covid was at a huge peak (I'll be honest & say I can't remember exactly when, because for a while every day seemed to be the same) my friends partner was really, really ill but the ambulance wouldn't take him to hospital...