West Malling is adjacent to the tracks taken by a/c departing LHR/LGW on DET SID's.
As Detling is only a few miles from West Malling, you are seeing most of the outbound traffic from these airports routing towards the continent.
Other traffic you may be viewing are STN departures routing via DET plus the LCY arrivals and departures...usually Bae RJ's or Fokker 50's.