I operated from Heathrow for thirty years and carriage of extra fuel for Heathrow was always discretionary.
I do recall there was an AIC (I think)many years ago which advised that extra fuel should be loaded to take account of holding delays. This was general advice not specific to LHR, though clearly LHR could be seen as an example. It may be that individual operators adopted extra fuel for LHR as a company policy, and this is what LEM is referring to.