1.8 million people squeezed into an area smaller than an airport

Can someone redo the image but outline what actually constitutes Heathrow rather than Heathrow-plus-surrounding-area. It makes it a much harder read, especially since the red is pretty cleanly laid over a rectangular area of the airport that makes it look like it's the whole airport.

My first thought was "Half? They're even labeled to the same size?" And took a second to realize that more than just that are is Heathrow.