Taxi
The usual black cab is available from all terminals. Look for the yellow TAXI signs as you leave the arrivals and enter the main airport. We are assuming you are going to central London, for a hotel or similar. Head for and join the queue.
The black cab will take you door-to-door, more or less. It is relatively hassle free and comfortable.
Unlike many airports, there is no fixed rate. Cost will depend on distance, number of passengers, length and time of journey and so on. It’s an expensive option. You might spend £150 or more getting to your destination (plus tip). This might seem reasonable if several of you are travelling together, you have lots of bags or other issues that make public transport difficult.
The journey is usually quick until you get into London, when you will be subject to road works, traffic lights, diversions, demonstrations and more. One way systems sometimes make it hard for the driver to get you exactly to the front door.
Give the driver the address as clearly and simply as you can. A postcode won’t really help as it’s just a region. A request like “The Grafton Hotel near Warren Street” will get you to the front door with no problem. If you don’t speak English, show the driver on a piece of paper or app with the booking. Drivers know most hotels, but some hotels have very similar names and it can be confusing (eg asking for the ‘Hilton’ is no help).
Heading back to the airport, you will need to know the terminal. Many drivers ask what airline, but Heathrow is still confusing.