Whilst we are all guessing, why would it not be the long hauls pre 0600?
Well you would have to ask Gonzo for an ATC view, but I suspect that:
a) a >3° approach may not be as feasible in a Heavy, compared to, say, a 737/A320
b) LHR operates with a single runway pre-0600 and interleaving steep and standard approaches on the same runway (assuming not all arriving types and carriers are certificated for 3.2°) might well present interesting challenges for ATC (and how would the ILS cope?).
Of course I'm only guessing.
