Transport For London have, in their wisdom, decided to start phasing out the 7 day travelcard. No timetable for the change has been settled but you currently cannot purchase one at a ticket machine except between 7.00am and 10.00am Monday through Wednesday. They can still be bought at ticket offices and elsewhere.
They believe that the computerised Oyster Card is quicker, cheaper (for whom?) and more secure. However you must pay a refundable deposit of 3GBP for one to go with a 7 day travelcard. It can be returned to any manned ticket office.
They assure me that they are aware of potential problems with regard to tourism and that they are looking at a tourist version of The Oyster Card.
One wonders if they plan to also phase out all paper travelcards although the gentleman I spoke to genuinely didn't know the answer to this.
They have promised to keep my updated and I will do the same here. You can get all the info about Oyster Cards at www.oysterdcard.com.
I would again stress that no timetable for withdrawl has been set and that for now, I will continue to recommend buying your travelcards (for whichever duration, 1,3 or 7 days) upon your arrival in London.
Hope that had covered everything but please do contact www.tfl.gov.uk if you have a query and you can also reach me at firstname.lastname@example.org