Mandrake wrote:As an alternative, if you buy an all day ticket for the Underground, wait until after 9.30am to buy, a lower price applies

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Its been a while since i've been to London. But you can buy a week long 'travelcard' can't you? I am sure it was slightly cheaper as well.
Again, like Mandrake said, get an off-peak one.
You can get an oyster card, and put a travelcard on it, so you have a plastic one instead of paper. Though if you don't plan using transportation much, then the oyster card is good. Not only does it give you the best fare, it is also so much easier than having to worry about tickets and change.