I'd always opt for driving but if that's not an option maybe get a train to London and then the Heathrow Express from there?? The Heathrow express runs every 20 mins and takes 20 mins to get there from either Paddington or Waterloo (can't remember which one). The other option is to get the Picadilly line Tube from London to Heathrow but it's about an hours journey due to the number of stops!!