As others have said, it depends a bit where in London you want / need to go to, it's a very big place.
Coming from Oostend my preference would probably be Southend or Damyn's Hall if a wet(ish) gras rwy is fine for you. Both have good connections to East/Central London.
One caveat about Southend - it is a proper airport with all that entails, i.e. hi-viz vests and security check when going airside - don't leave your Leatherman in your bag!
Another option, especially if going to the western parts of London would be Denham. Essentially you would fly along the northern edge of London until more or less abeam Heathrow. Hard rwy (although parking on grass, IIRC) and close to a train line running to Marylebone station.
Have fun!