I wonder to what extent the maintenance etc arrangements of BA are in some way allowed to override the use of stands for arrivals and departures - and the maintenance base being about a 3 mile tractor drag from the terminal does not help.
Way back in the old Terminal 1 days I recall more than once BMI complaining about BA having aircraft on stand for extended periods, to the extent of doing engineering work while there, with others waiting for a stand.
Found a slightly old doc
here with one or two interesting stats in it :
The "pier service" target BA have set for T5 is 95%.
and
AOC explained the reasons why BA chose not to put some of the last wave aircraft on stand, that if maintenance of long haul aircraft did not take place on stand it would require substantial amounts of towing with adverse consequences to airfield congestion and the maintenance/engineering constraints had been applied for many years
So yes WHBM, it does look like "special" arrangements is what means BA can have a pier service target of 95% when I believe its 99% for the rest of LHR.