A level results day - when are grade boundaries published?

As described above

On A level results day itself. For example, last year they were published on 18 August 2022.

They should be on-line first thing in the morning - before you pick-up your results from school / college - so you can tell whether you've just missed a grade boundary and want to immediately request a "review of marking".

Ah okay thanks. I was just wondering because I’m planning to look at the grade boundaries while waiting for UCAS/websites to load

You won't have your grades / marks at that point, so knowing the boundaries won't be enormously useful. But it'll while-away the time, I guess.