Originally Posted by UAkls
There are two options to dowtown London. The fastest is the heathrow express (
http://www.heathrowexpress.com/), which takes only 15 minutes but will cost almost $50 roundtrip (25 GBP). It will take you to Paddington station, which is then just a short cab or tube ride to Picadilly or Trafalgar. You can also take the less expensive Underground (about $15 roundtrip) but it will take 45min-1 hour central London (see
http://tube.tfl.gov.uk/). So, it is a cost/time trade-off.
Two things: -
1) The 'loop' on the Picadilly line is closed at the moment meaning to use the tube, asssuming you're flying BA into T4, you need to take a bus/taxi from T4 to Hatton Cross for the Piccadilly line into town.
2) The good thing about the Piccadilly line is it takes you straight to the "touristy" centre of London. The Heathrow Express takes you to non-descript Paddington while also being expensive!
http://tube.tfl.gov.uk/