Airport services don't make 'the profit', as you call it, alone. They make part of the overall profit for the whole company from which everyone benefits. For 2005, NERL made an operating profit of £114M, Airport Services made an operating profit of £10M.
This came after offsetting an operating cost of £330M for NERL and £82M for Airports. So buck for buck, NERL run a tighter ship when it comes to profit against costs spent. Or a better return if you want to put it that way.
Or they overcharge massively and CP2 is to redress that while NSL is more commercially realistic and has been for many years
