anotherthing.... The airspace around Biggin has controlled traffic even when Heathrow has no traffic. A large aeroplane lurking around would be noticed by Heathrow or Gatwick or TMA controllers, of that I have absolutely no doubt. If such an aeroplane were to fly in that area without making contact then controllers would presumably be taking avoiding action against it as it would constitute "lost or radio failed" and that would lead to an investigation.
I'm inclined to think this thread is getting rather silly so I'll contribute no further..