The Northern Line (Black one)
Take Northern Line northbound to Leicester Square, then change for the Piccadilly Line northbound to get to King's Cross St. Pancras. I recomend that a change at Leicester Square is easier than at Euston.
Take the Piccadilly line from Heathrow to King's Cross St Pancras (St Pancras is the Eurostar station).
From Waterloo take the Bakerloo line to Oxford Circus. There transfer to the Victoria line to St. Pancras International rail station. Journey takes about 20 minutes with one change of train. Check the 'London Tube' site for service delays and restrictions.
The most direct path is east to the A201 south, across the Blackfriars Bridge to Southwark and then "The Cut" west to Waterloo Road. It is 6 minutes faster, on average, to use the A4200 due south across the Waterloo Bridge.
As there are no direct trains from Heathrow to London Waterloo, it's quicker to travel by underground from Heathrow on the Piccadilly Line to Leicester Square Underground Station and then change onto the Northern Line to London Waterloo Railway Station.
The journey time at morning peak is between 25 and 30 minutes, dependant on whether or not you are prepared to change train enroute.
Take a train from London Waterloo station.
Not directly, you will need to take the South West trains service to London Waterloo and then you need to take the Bakerloo line tube to Piccadilly Circus, and then you can take the Piccadilly Line all the way to Heathrow Airport.
Yes, however not directly. The quickest way is to take a South West Trains service from Guildford to London Waterloo. From London Waterloo, take the Northern Line to London Euston. From London Euston, take a Virgin Trains service to Preston. From Preston, take a First Transpennine Express or Northern Rail serivice to Blackpool North. Total journey time is around 4 hours.
You can take the Eurostar from Paris to London St. Pancras.
No, not directly, you would need to take the train to York and then take the King's Cross train into London, from King's Cross/St Pancras you can then take the Piccadilly Line tube to Heathrow.
London St Pancras International - Paris takes about 2hr 15 min