There is a preference for 27R arrivals (in the said time frame) for operational reasons since the majority of landings at the time are coming off either Lambourne or Bovingdon holds.
That said the Kenya and few T4 long haul carrier may get 27L as may the QF/EK A388s for operational reasons.
If we're still talking about the 04:30-06:00 timeframe here, then I don't see much evidence of any preference, at least not recently.
Since the beginning of April, I can only count around 10 landings that haven't conformed to the published night time alternation programme.
That said, in the weeks when the southern runway was the designated one, there were, on average, around 2 fewer landings daily between 04:30 and 06:00 than there were in weeks when it's the northern runway.