The
Airports Commission report, which recommended construction of a third runway at LHR (the "Heathrow Northwest Runway scheme"), had the following to say on Northolt:
11.41 For the Heathrow Northwest Runway scheme, advice from NATS has identified a high likelihood that the new runway would have significant operational impacts on RAF Northolt, a military airfield located six miles north of Heathrow, which also accommodates a number of civilian business and general aviation movements. While the scheme would not require the end of military movements at Northolt, there is a significant risk that it might not be possible to continue to operate civilian flights from it without some impact on the capacity of the scheme (potentially on a one-for-one basis, reducing capacity by up to 7,000 ATMs).
15.14 Some respondents also argued that greater use of RAF Northolt, coupled with improved surface access links between RAF Northolt and Heathrow, could facilitate domestic connectivity. The Commission has not taken a position regarding the future use of civilian capacity at RAF Northolt. It has not, however, been convinced that there is a credible solution for providing a transfer service between RAF Northolt and Heathrow or that RAF Northolt is a viable long-term option to address Heathrow’s capacity constraints.
Until the government announces where the new runway will actually be built, how can any offer of capacity at Northolt can be made to Flybe without giving the impression that the recommended Heathrow option has been rejected? The cost of modifying infrastructure at Northolt could only be recouped over a medium- to long-term basis; based on the comments above, this would probably not be viable if the Heathrow runway was built. The runway issue is so toxic among the Conservatives that I can't see any MOD ministers wanting to get dragged into the debate by throwing Northolt into the mix.
Northolt is a military airfield first and foremost, for example being closed to most civil traffic when Typhoons were based there during the Olympics. Business aviation can cope with such disturbance by using alternative airfields at practically no notice, making it the ideal customer for Northolt's spare capacity. Scheduled airlines not so much.