Not sure why you think it's the busiest airport in the world
I suspect because the BBC and other UK media outlets used to frequently to refer it as being so, then somewhere in the piece/article they would quietly slip in the qualification that they were talking about it actually being the busiest passenger airport in the world in terms of international passengers (how many passengers passing through ORD or ATL have to have a passport...).
Not sure that's true either nowadays, think the likes of Frankfurt, Schipol aren't far away ( and UK passenger duty is possibly driving lots of connecting traffic away from the UK anyway, but that's a different story)