Windsorian, where are all these nothing-but-heavy aircraft going to park? Many stands at LHR are for small aircraft. Taxiway restrictions abound for the larger aircraft types, especially A380s. You might find that if you resize all possible stands, you just don't have the capacity to cope with 1380 movements a day.
I would argue that 100% load factors are not desirable, as that shows the airline that they have underpriced the tickets. Ideally, you'd want a few empty seats on each flight to prove that the pricing was just ever so slightly high to fill up the aircraft. If every seat was full you would have no idea what prices the market could take.