There's no link between all the systems being used. When we came back from Gibraltar, filled out the landing card thing on the gov website, but still had to show it at passport control in the UK as they don't have it on their systems. Don't scan it or anything just show it.
We both then had covid shortly afterwards. Filled in all the track and trace forms (about 45 minutes!), asks if you've been on a flight but doesn't ask the flight number.... Then had a call from track and trace to 'confirm' what we put on. Basically wanted me to tell them everything I already had online. When I questioned this they said they can't see the answers for the online form! A couple of additional bits they asked which weren't on the form were on the landing document, which again they can't access...
Long story short, it's impossible for them to track people when agencies can't even see their own forms. So it would be so easy to get in from anywhere with quarentine