Heathrow is London's largest airport and the busiest in Europe by passenger traffic. It is approximately 17 miles west of central London and well connected to the centre by Underground and mainline trains. Local buses also connect the airport with various places in West London including Harrow, Hounslow, Feltham, Southall, Uxbridge etc. National Express run coaches from the airport to various destinations around the country, including most of the other London airports.
North West (M40 towards Oxford, Birmingham, Manchester, Glasgow)
Take the London bus 140 (direction Harrow Weald) to the "Target Roundabout" bus stop. This will take around 30 minutes and cost £1.50. You will need an Oyster or contactless payment card. From the bus stop walk a very short distance on the same side of the road past the Star Wars shop and then to the left, past McDonalds. You will see an ASDA petrol station. Traffic from here has to go onto the A40 which leads to the M40 (and M25) a few miles ahead. It would be recommended to hold a sign saying "M40" or your final destination, possibly "Warwick Services" if you plan on going past Birmingham as that is the last service station going north on the M40.
Public transport to central London
The London Underground Piccadilly line will take you directly from the stations at Terminal 1,2,3 / 4 / 5 to the centre of London via Hammersmith and Kensington in around 50 minutes. It connects with many other lines.It is the cheapest option. With an Oyster or contactless payment card the fare to zone 1 is £3.10. Between 0630 and 0930 Mon-Fri a peak time fare of £5.10 applies. If you pay by cash the fare is £6 at all times.
Two mainline railway service also operate from the Terminal 1,2,3 and 5 stations to London Paddington station. The Heathrow Express is a non stop service taking 15 minutes to get to Paddington but is incredibly expensive (£22-£25 single.) The Heathrow Connect service also takes the same route but takes 30 minutes and makes 5 stops including Ealing Broadway. It costs £10.30 single to Paddington (£8 to Ealing Broadway.) As it is also quite expensive it would be recommended to take the Underground instead unless you are going to somewhere on the route and are in a hurry.
If you plan on travelling to Croydon then you can take the X26 London bus, avoiding travelling into the centre of London and getting there for £1.50. While the X26 is an express bus it is slow, taking just under 2 hours and potentially longer at busy times of day.
Public transport within the airport
A free travel zone operates around Heathrow airport. You can travel for free between the terminals on either buses or the Underground/ Heathrow Connect/Express trains. It is also possible to travel to Hatton Cross Underground station for free.