London is the world's top destination for air travelers.
It is set up as few other cities for economical travel. There are six major airports in the region that serve hundreds of commercial flights a day. London City Airport is within a few miles of the city center, while Southampton is 125 kilometers (76 miles) from the center. Chances are good you'll land either at Gatwick, Heathrow, Luton or Stansted.
Special offers are frequent for London-bound travelers, especially those leaving from the United States or Canada.
British Airways sometimes offers low winter fares to London from U.S. cities. It's worth checking their current offers, especially at that time of year.
Iceland Air has long been a budget airfare leader, and not just to Reykjavik. You can sign up for email delivery of their for Icelandair's Netclub to see their latest fare offers. Another site worth checking is Virgin Atlantic. Although it doesn't offer trans-oceanic flights, budget carriers such as EasyJet offer good airfares to the European continent and nearby Ireland.
North American air carriers offer choice fares to London, too. There's no way to say which ones consistently come up with the best deals, but you can check their special offer pages here at About's Budget Travel.
Are We There Yet?
There is no use finding a low airfare to Gatwick, and then spending $150 USD or more just to get from Gatwick to Central London. Yes, that is possible--via one of those fancy, delightful Black Cabs for which London is famous.
You're not really "there" until you arrive in the city.
Start by checking the airport Web site appropriate to your itinerary. Gatwick, Heathrow, Luton, London City Airport, Stansted, and Southampton all maintain sites with terminal diagrams as well as information about trains, planes and automobiles.
If you enter London from another country, keep in mind that airports frequently are lousy places to exchange currency. Banks in your hometown are unlikely to be the most favorable places either, but they'll give you a better rate than the airport changers. The best advice is to change a small amount to British Pounds at home, then use that amount to get out of the airport to better rates in town.
How much money will that take?
There is a Heathrow Express train that takes you to Paddington Station in Central London. It's fast and £16.50 ($27 USD) when you purchase a one-way express trip online. That's about twice as much money as riding the Underground. Estimated "Tube time" from Heathrow is about an hour, but you'll want to allow more than that if you're catching a flight.
There is a similar Gatwick Express train to Victoria Station.
Given the volume of connecting flights here, chances are at some point you will experience a London layover. If you have about eight hours, it's worth exploring half-day budget travel itineraries in London.
More information about airfares is available at About United Kingdom for Visitors. Do you homework and save money you can use for things that are far more fun.