Buy an Oyster card at the staion and pay a five Pound deposit.
You then prepay your expected transit use including trains, Metro (tube), and surface buses (the red double decker)
The card saves money on each trip and more importantly it saves you from waiting in line to buy tickets.
Swipe the card on entering the station and swipe again when leaving the station. The machines will indicate the credit remaining.
Swipe the card only once on the buses, when entering.
Top up at a sales window or machine when credit gets low.
Redeem the credit on the card and the card deposit when leaving the country.
Note: Redeem early because not all windows will redeem the card.
The Oyster card saves money and automates the system. If you use the Metro, bus, or trains on a London vacation,
the card will save time and money.