Overview
London is served by six commercial airports: Heathrow LHR, Gatwick LGW, Stansted STN, Luton LTN, London City LCY, and Southend SEN. They serve different parts of the market rather than functioning as one integrated hub.
Airports
Use Heathrow for the broadest global network, Gatwick for a strong mix of leisure and long-haul, Stansted and Luton for low-cost European flying, London City for fast access to the City and Canary Wharf, and Southend when its smaller route network happens to fit. For a decision by destination inside London, use Choosing the Right Airport.
Planning
The airport is only half the trip. Compare Ground Transport, check early and late flights, and use Apps and live services for live transport information. If you are curious why London ended up with this fragmented system, see History of London's airports.