Mostly this is now down to airline checking. In essence what goes on at the UK Border is to check that airlines are doing their job, and when they do spot checks it is really "check the checks". So the short answer is at check-in LAX for sure, and maybe LAS if they are on the ball there.

You don't need a PCR for the UK, NAAT, RATs and LAMP will all do, so I guess this is for Jersey.