The company (the Reuban brothers) who own Biggin Hill also own London Oxford at Kidlington, therefore have most to gain, by Northolt not being able to accept civil flights.
It only mentions runoff areas, but this seems an odd difference to pick on. It seems Biggin itself is rather close to the A233 at one end anyway. Pot and kettle? Is Northolt really any worse than Southampton in this respect?
Look at google maps for the run off at their other airfield at Oxford.
By this ruling they will control 66% of the biz jet traffic for the London area, Farnbrough is a little too far away. I suspect the MoD will appeal this decision.