Due to London's huge global city status it is the most served destination in the world when it comes to flights.
London (all airports code: LON) is served by a total of five airports. Travelling between the city and the airports is made relatively easy by the large number of public transport links that have been put in place over recent years. However, if transiting through London, be sure to check the arrival and departure airports carefully as transfers across the city may be quite time consuming. In addition to London's five official airports (of which only two are located within Greater London), there are a number of other regional UK airports conveniently accessible from London. Since they offer a growing number of budget flights, choosing those airports can be cheaper (or even faster, depending on where in London your destination is).
From the USA:
There are direct cheap flights from various airports in the USA to London arriving at the London City Airport (16km from the West End), Heathrow (24km west of central London) and Gatwick (45km south of central London).