You have to be realistic.
North Weald is a pure VFR field with no IFR procedures and no licenced ATC. It has many things going in its favour as a recreational aerodrome but it would have to improve its facililties dramatically if it were ever to compete in the corporate market. (which the owners do not want to happen).
Southend can hardly be called London. It can take BBJ if they are operating empty and they have a good trade in B737 maintenance positioning empty. If they ever get the go ahead to extend the runway it might be more attractive.
The OP is looking for somewhere to operate a Citation so you can look outside of the normal destinations to places like Blackbushe or Cranfield. But in the end if your pax want a top level FBO near London then stick with the old favourites of Farnborough, Biggin Hilll, Luton, Northolt and City.