Of course Ocean Sky or Signature could also help you at Luton(!)
Farnborough isn't that cheap either and Biggin is a pig of a place to get anywhere from. Even though it is slot-less and 'cheap' it has not had a great success in attracting business aviation compared to the others.
Northolt is movement restricted and could probably fill its civil slot allocation several times over, it is a vastly underused facility that could make the govt a lot more cash if things were relaxed a bit.
If you are wanting to access east London then Stansted or Southend is a possibility. Even Gatwick seems to be ramping up its GA visitors.
Since arriving at Heathrow Signature have done a great job ramping up operations and finding available slots compared to a few years ago when business aviation was almost never seen. I still believe that if the slots were available many more operators would prefer to use LHR than the more distant options.