Is there (these days) any possible good reason for aircraft to arrive in the Heathrow area so long before 06:00 that they need to hold? Today there was an AA 777 holding at 05:30, joined by two BA examples that took a long route to finals and a QR A350 that did one quick circuit. It must cost the airlines money to keep planes and crew in the air for an extra 30 minutes, and I cannot believe it's beyond the competence of an airline operations dept or pilot to arrive within an airport's opening hours.
It's not a case of "opening hours" (Heathrow gets arrivals from around 04:30 onwards daily), but there is a Night Quota governing arrivals between then and 06:00 and if your flight isn't on the list (or exempted) then the rule is that you don't get to land before 6.
That said, there have been plenty of instances of flights with an STA after 06:00 having been allowed to land early in recent years, so I don't know what determines the decision on an individual flight basis.