Starliner
This is what our airline rule book says...
Flights into LHR and LGW with a scheduled arrival time of 0615 Local or earlier must not land before 0432 Local. Other flights must not land before 0602 Local.
Its also explains
Airlines are permitted to schedule movements (both departures and arrivals) and operate delayed flights in this period up to a set maximum of both movements and noise quotas. Any exceedance will normally result in a reduction of the allocation for the next season. An overrun of these limits by more than 20% is not permitted and no more movements would be allowed, during the “Night” period, until the end of the season.
So what you are seeing is the flights with the scheduled arrival of 0615 or earlier going straight in to land, whilst those with a later arrival time join the hold and await the end of the night limits at 0600.
Always a good game of trying to work how early do you want to get to London. Get there early, hold for 15min, but be at the front of the queue for landing, or get there at 0600 and find you are at the back of the queue.
What also tends to happen is the available night slots start to get tight towards the end of the season and Ops will then ask the crews to land after the night jet ban time. This year has been quite good - with the ash cloud closure there has been a fairly healthy surplus of night slots available.