If you wind the clock back to 1946 where else near London would you build an airport-
South and North are very hilly and to the east its all docks and mudflats and weather that would make operation of airliners of that era a challenge (ie its always foggy)
So LHR is a good almost the only choice for a big airport because its the only flat real estate for along way . The problem is that they should have aquired the land north of the airport to make provision for R3