London Heathrow Airport is one of the largest in the world, handling the biggest portion of international traffic of any airport globally and ranking as the busiest in Europe. Annually the airport sees close to seventy million passengers, of which only around ten percent travel to domestic locations. Heathrow has five terminals and a third runway is currently undergoing planning, so it is set to become even busier. With terminal five having only recently opened, it is advisable that you confirm with the airline you are flying with which terminal you will be landing at, particularly if you are intending to hire a car from one of the companies based out of the airport. At present BMI operates out of terminal one, British Airways from terminal one, four and five, and Virgin Atlantic from terminal three. Many other international airlines operate from terminals one through four.
The best way to explore the United Kingdom is by car and all but one of the terminals in Heathrow offer car-hire facilities. Avis, Hertz, Europcar, National/Alamo and Enterprise car hire companies all operate out of terminals one through to four; Budget has desks in terminals two through to four; and at present there are no car hire desks at terminal five.
It's important to note, however, that all the terminals are connected either by walkways (as with terminals one, two and three) or by free public transport (as with terminals four and five). Once out of the airport complex you can easily navigate around London (without having to go into the centre of the city) by using the M25 ring-road.