I was looking to see how much an Off Peak One Day Travelcard cost from Dartford into zones 1-6 and found that the normal off peak version is reasonable at £12.80. But next to the £12.80 fare is another Off Peak one Day Travelcard priced at £19.90 - The difference between the two is that the £19.90 is valid on High Speed. Upon checking the RSH Journey planner for this particular ticket it seems to allow you to travel AWAY from London down to Gravesend before changing to HS1 into St Pancras. This strikes me as being rather odd. Being able to travel away from the London Travelcard zones!