You will probally find the the gatwick routes are protected as established operators do not want to lose slots. At the moment slots are not so much of a problem at LTN and STN but as LGW fills up as it was doing before when ba expanded at the airport then further charter expansion will be pushed out to the secondary airports.
I also think that the charters are starting to cut back in response to the loco's and this is across the uk not just in th SE or NW.