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.
I see no option but to assume it's a deliberate choice by someone. But why?
More than likely they just had a good routing or favourable conditions and arrived early.
If you drive non stop from Scotland to London it would be hard to arrive bang on time .