Back when I 'served' you all at London's Heathrow airport it was the busiest in the world, but after I left it went into severe decline and so has since been overtaken as Europe's busiest by Istanbul (above). It might change with a third runway, although as this was first considered eighty years ago your breath should not be held. We do have the consolation IST was designed by London architects... ours being a service economy.
Actually LHR once had a third runway, the crosswind 23 that I used on windier days altogether. Without an ILS, it required a sidestep that would put the fear of God into most airline pilots ~ as it did me.

