When I was doing a lot of LHR-ABZ last year on BMI when they were still using A320s some of the woefully thin loads, even at classic business times, were surprising. The majority of my flights then would have fitted into an Embraer RJ no problem.
If the general London to Aberdeen market were so good then Easyjet would have got beyond the two a day to Luton and nothing to Gatwick they currently manage.
For those who believe that train is the way, I find that for all the business people I meet in Aberdeen or travel there with, Heathrow, Gatwick or Luton are infinitely more convenient at the London end for them than Kings Cross. It's a myth that the majority of travellers want to go to the heart of major metropolitan areas any longer - not that Kings Cross is particularly in the heart of things, unless you're going there for the hookers in the semi-derelict houses around the station.