Aerospark, I have always assumed they are held low when necessary to get underneath the Epsom hold and occasionally the Biggin hold, and the traffic from both routing to join the approach. Presumably Gatwick traffic also can have an effect on outbound altitudes and routings which seem to vary considerably throughout the day.
Incidentally, I was awake very early this morning and had a squizz at Flight Tracker; seemed to be more than usual amount of heavy traffic in all four Heathrow holds; wonder if there was a delay or perhaps some of the trans-Atlantics had very favourable winds...