Say you want to take advantage of the Eurostar train from London to Europe, how do you get from Heathrow to the train? Aside from booking a limo, if you want to use public transportation, take the Underground (details here) to Kings Cross / St. Pancras Station, for 4 GBP, and then take National Rail the rest of the way nationalrail.co.uk for 23.70 GBP. Prices as of 18-Apr-2009.