You could also take a black cab - but that will be very expensive - the last time I did it, it was over 50 pounds - but if you can find people at the airport or are travelling together, then it pays to go as a group and reduce the cost by splitting the fare - and since there is a bus/taxi lane into London, the ride is very fast and the cabbies are incredibly knowledgeable about the City itself. almost 7 years ago
here is some tips:
1) Fastest: getting to and from Heathrow Airport - BY RAIL
a) Heathrow Express is a direct high-speed train connecting the airport and London Paddington Railway Station (where you will also find Paddington tube station on Bakerloo line). Trains run every 15 minutes, the trip to London Paddington station takes about 15 minutes. For prices and further information, please see www.heathrowexpress.com. Tip: Tickets purchased online are discounted (without discount ticket costs around £18, discounted ticket = £16.50).
b) Heathrow Connect: stopping service to London, calling at local stations in west London and reaching London Paddington Railway Stationin 26 minutes. Ticket costs around £8.50. For more information please visit www.heathrowconnect.com.
2) Cheapest: getting to and from Heathrow Airport - by UNDERGROUND
London Underground's Piccadilly Line provides the most cost-effective rail route between Heathrow Airport and the capital. Journey time is under an hour and you shouldn't have to wait longer than ten minutes for a train, even off-peak.
Heathrow has three London Underground stations, all are in Travelcard Zone 6. A single fare to central London costs approximately £5. If you purchase a Travelcard and you're covered for London buses and some mainline trains too. Tickets are available at all London Underground stations, from ticket offices or machines. Pre-paid Oyster cards can also be used on the London Underground.
3) Door to door: getting to and from Heathrow Airport - by BUSES and TAXIS
Direct bus service operates between Heathrow terminals and a large number of hotels near the airport: "Hotel by Bus" ( www.hotelbybusheathrow.com).
Heathrow taxis, minicabs and chauffeurs... click here: www.heathrowairport.com/transport-and-directions/taxis
Independent A-Z guide to London, England and Scotland (http://www.mintytrips.co.uk) about 6 years ago