Slaphead/all,
That is indeed correct, there is a total number of permitted civilian movements pa but Northolt are allowed to handle as many Military/government related movements (not just UK mil' but any Military/Government connected aircraft).
There are also very tight operating guidelines for Civilian operators (limited opening hours, usually something like 0730-2000, mon-fri, closed weekends & bank holidays, no single engine aircraft etc..). However, Netjets lease a hangar at Northolt so are classed as a 'based operator' so they can get around the official opening times (to civilian aircraft) & operate to & from Northolt at weekends (along with any mil'/gov't related flights)
HTH
Fried Chicken