"John B" typed

You also need to think about how much travelling you're going to be
doing - an Oyster card comes in two varieties. It can either be a
Travelcard (a season ticket, in your case lasting a week) or
Pay-as-you-go (you put cash on it and then get money deducted per
journey).

If you're going to make more than about two Tube journeys a day within
central London, a Travelcard is a good idea; if you're going to make
fewer than that it will probably be cheaper and easier to get
Pay-as-you-go.

If you do not use National Railways, the amount you spend on a
Pay-as-you-go Oyster should be capped at 50p *less* than the equivalent
Travelcard.

Cash Tube and bus fares are about twice as much as on Oyster PrePay so
its worth getting one.

A PrePay Oyster (without a season ticket loaded) does not expire with
time and may legally be passed on to any friends who visit London.

However, Pay-as-you-go is only valid on some ex-British Rail train
services (and is not valid on any ex-BR services from Wimbledon),
although it is valid on all trams, Tubes, buses and DLR trains, so if
you plan to use those then it complicates the picture again.

--
Helen D. Vecht:
Edgware.