Cost !
The operation at Northolt is in money terms very small in comparison with the Nimrod project, the aircraft are cheap , parts are cheap and Netjets is paying a large part of the cost of the airfield.
The Nimrod is hand built , packed with very costly kit and takes a lot of people to fly and maintain it.
So all in all I dont think selling the Northolt operation would pay for 2% of the Nimrod operation.
That is not to say that I don't think scrapping the Nimrod was a very silly thing to do.