Visitor Oyster Card
There is also a Visitor Oyster Card. This is £5.
You can on buy this on-line, before you leave home, and have it delivered to you. You cannot register it on-line and you cannot buy it when you get here except at some airports and on the Eurostar.
There are some locations in other countries where you can buy this card. Since you will have it as you enter the country, you will not have to queue for a ticket at eg Heathrow.
These cards are not refundable and are disposable, though it may make a cheap souvenir of your holiday. However, there is a daily spending cap on the card.