Leon, Agreed.
Most military "airfields" these days have more than one military purpose. Putting a cost on running them is virtually impossible. Does unit A, which is part of a separate command get counted or not counted? Unit B is a lodger unit. Unit D is under part command and is only at the airfield until new infra is in place at X in 20xx. It's comparing apples with pears, Ahem! Northolt's advantage is its geographical location.
However I have little doubt that the government will f..k up the legalities and the tiny bit of cash Northolt makes will be lost. Those operating into Northolt will have to go elsewhere and everyone loses. Sorry; cynical tendencies coming out after 3 of Hook Nortons finest.