I completed my PLF for my original flight itinerary, however due to a DELTA flight cancellation, instead of flying LAX-NYC-LHR, I was flown LAX-AMS-LHR. No one checked in PLF, and connecting through Amsterdam was painful, as they need to "verify" your COVID test results, and PLF. The lady checking mine, did neither - didn't even look at the electronic COVID test, and the PLF was 100% wrong, as it was for my original itinerary. It was pretty bad.
The Day 2 COVID test on arrival at Heathrow was also confusing, as I booked it for LHR T3 (arriving on DELTA), but since i arrived with KLM, I was at T2. However, I did ExpressTest, and they were great, I went in, did the test, and it was absolutely fine, even though the website said they had no availability. Basically, it left me with no confidence about anything you read on-line, and just need to chance it and hopefully it will all work out, as no one has any idea what's happening.