HD has pretty much covered most things, but just to add some other info:
Civilian movements at Northolt are capped by the government (Think it's around the 7k/year mark), but there are no restrictions on Mil/Govt sponsored flights.
Landing fees for MoD airfields are set by the MoD as per Joint Service Publication (JSP) 360. Northolt do charge higher fees, but that is set to be competitive with the other London airports rather than trying to undercut them (we like to make cash too).
Runway configuration/airspace issues will always be a problem. a 40 deg dogleg at 4nm from TD on Rwy 07 is sporting on a bad weather day as Northolt only has SRA or PAR approaches available (no ILS, as you can't bend it). Any increase in movements when on Easterlies causes more work for Heathrow's INT Directors. If LHR get their 3rd runway then they would have to effectivly loose a landing spot on the new 09L in order to vector a Northolt 07 arrival. BTW there is an onging study into the potential of an ILS to 07.... with a few airspace changes
Building a new runway south of the A40 would cause the same airspace prediciment as having a 3rd at LHR - minimum horizontal seperation would be lost (currently 3.2nm between LHR 09L and NHT 07).
Re-alligning Northolt with LHR could potentially work - increase length at the Ichkenham end, but it too has it's own airspace issues (SID/STAR's)
Re-activating Rwy 12 instead of using 07 - LDA would be around 4700', which is bloody short, so big jets are out - not to mention the fact that a postal sorting office has been built at the end of it!
The short of it - IMHO - Northolt could handel more flights, (if it were permitted/given government funding), but to do this at the level they are looking at to get the 3rd at LHR, I don't think it's possible.
Also, as HD said, "have you tried driving through Hayes at rush hour", traffic around the Polish War Memorial junction of the A40 is far from ideal at current levels, let alone if we expanded that much!
MM