London Aiports
London is one the most served destinations in the world. To handle all those passengers, the city hosts 5 airports; London City, Gatwick, Heathrow, Luton and Stansted (and 2 smaller ones without scheduled services). London also has one of the biggest public transport networks within its boundaries. That doesn't mean that you should consider changing from one of London's airports to another. Just avoid that! To comfort passengers while stuck in traffic jams, some buses even have toilets on board. That says enough. If there's no way to get around a change of airport; plan a couple of hours for that trip and enjoy sight seeing.
Berlin Airports
After closing down Tempelhof, Berlin was left with 2 airports; Schönefeld and Tegel. Tegel Airport, the main international airport, is planned to be be shut down too, but only after the new Berlin Brandenburg International Airport is opened near Schönefeld Airport. The latter will somewhat be swallowed by this new giant.