I just checked the tfl.gov.uk site as well as the visitbritainshop.com.

TFL say:

Travelcards are paper tickets that can be purchased for various combinations of dates and zones. They are available for adults and children and do not need any photographic ID to be used for travel.

If the zones you require are not available from this shop, please purchase your Travelcard on arrival in London.

Please note that if you buy a 7 Day Travelcard from a TfL outlet upon arrival in London it will be issued to you on an Oyster Card.

Both sell it for the same price, and both will ship overseas prior to the trip with a paper ticket. TFL say that if bought at a TFL shop in London then it will be loaded onto an oyster card. I'd prefer it to be on an oyster card because the oyster is more durable and less likely to get misplaced.