London is serviced by two major airports: London Heathrow (LHR) and London Gatwick (LGW). Heathrow is the closer of the two, but both have easy access to London via public transportation. You can take a taxi from either of the airports (although Gatwick will cost more), but the trains that run from each of the airports may be your best bet depending upon how many people are in your party. There are a ton of public transportation options, so shop around before you go to know what your best option is--it could save you a lot of money!
Taxis can be expensive from either airport into the city. From LHR, a taxi ride will take you approximately 30-60 mins depending on traffic and will cost you anywhere between 70-100 GBP to Central London. From LGW, a taxi ride will take approximately 60-70 mins and will cost you around 120-150 GBP to Central London. To get an idea of what a taxi ride will cost to your final destination, you can try www.taxifarefinder.com.
Note: Private cars can sometimes be booked for SIGNIFICANTLY cheaper if you shop around prior to your trip. Additionally, UBER is currently still operating in London. Uber was stripped of its license to operate in the city on 30 Sept. 2017, but they have appealed the ruling--which should drag out through most of 2018. While the appeals process is going on, they are continuing to operate as usual in and around the city. If you pre-book an UBER (which can be an amazing value in some cases when compared to taxi rates), watch out for potential surge pricing.
The Heathrow Express: From LHR, this is probably the best way to travel depending on your specific circumstances. Trains arrive in Paddington Station in the west end of central London. To find the train in the airport after your arrival, simply follow the signs for "Trains" or "Heathrow Express." The trains from Terminals 2, 3 & 5 are direct to London. Terminal 4 has an inter-terminal transfer. The ride is only 15 minutes from Terminals 2 & 3 and about 21 minutes from terminal 5. From Terminal 4, you must change at Heathrow Terminals 2 & 3. Mon-Sat, the first trains from Terminals 2 & 3 depart at 5:12am; from Terminal 5 5:07am, and from terminal 4 the first inter-terminal transfer departs at 5:23am. On Sundays the first departure from Terminals 2 & 3 depart at 6:05am, Terminal 5 at 6am, and Terminal 4 at 5:53am. The trains begin departing Paddington Station for the airport at 5:10am Mon-Sat and 6:10am on Sun. The train departs from both the airport and Paddington Station approximately every 15 minutes until 11:25pm from Paddington Station and approximately 11:40pm from the Airport (depending on the terminal). The most recent timetables for both directions can be found here.
Children 15 and under travel free in Express Class when accompanied by a paying adult (or unaccompanied with proof of air travel and age). In Business First they may only travel free if accompanied by an adult who has bought a Business First ticket (not unaccompanied). There are many discounts including one for groups of 3 or more adults and advanced purchase. You can find more information on discounts here. You can book your tickets ahead of time at www.heathrowexpress.com, directly at the station, or even on board the train. You can find the most up-to-date pricing for tickets at the website, as well or directly here. From Paddington Station, you can easily get to your final destination in London via the tube, a taxi, or another train.
The Gatwick Express: LGW Airport has 2 terminals: North Terminal and South Terminal. The railway station is in the South Terminal, a short walk from arrivals. If you fly into the North Terminal, there is a free inter-terminal bus to the South Terminal/train station which only takes two minutes and runs every few minutes 24/7. The train goes into Victoria Station in London and takes 30 minutes (35 minutes on Sundays). The first trains depart LGW at 5:51am Mon-Fri, 5:45am on Sat, and 5:50am on Sun. They run every 15 minutes until 12:50am, and the final train is at 1:35am. From Victoria Station to LGW, the first trains depart at 3:30am and 4:30am then every 15 minutes until 10:15pm, and the final train is at approximately 11:20pm. The most recent ticket prices, timetables, and discounts can be found on their website: www.gatwickexpress.com.
Heathrow central bus station is located between Terminals 1 and 3 and is open 24 hours a day. National Express and other coach operators run services from Heathrow to more than 500 destinations. There are buses to rail stations, local destinations, and even many of the London hotels. For more information on Heathrow's coach and bus network, click here.
Bus service connects Gatwick with more than 400 destinations. Direct services run to destinations including Heathrow, central London, Bristol, Brighton, Oxford and Cambridge. Most bus service operates from the South Terminal, but some does depart from the North Terminal, as well. More information on coach service from LGW can be found here.