To be honest London is a central travel hub for people coming from outside of the UK. There are other international airports, but what you save from not flying to London is then often made-up by having to pay for car rental/trains etc on top.
Like when I go home from Ireland, I can fly to a regional airport and it's substantially less than flying to London. But after I factor in trains etc on top it ends up being a much longer journey for pretty much the same amount as flying directly.