Just as a reminder .... if you are travelling around London, do so as cheaply as you can by paying the lowest fares available. An Oyster Card which is a pay-as-you-go, prepaid card to almost all of the capital's public transport. That includes the Tube, London Overground, buses, tram and the Dockland's Light Rail. These smartcards are easy and convenient, and intending foreign visitors can buy them even before the leave home. To find out more information, check the link -
Oyster Card.