My family and I are traveling to the UK in June. There seem to be a lot of options for as far as card types, and when to purchase. We need to take the underground from Heathrow to St. Pancras. We will be in London for about 5 days and plan on using the underground and buses to get around. Is the Travelcard the best option, or should we get the Oyster card? Should we buy the card prior to arriving, or after we get there?