njc,
from your other threads I see that you will be traveling on your own, and that you are considering a day trip to France.
I don't see any advantage to you to consider paper Travelcards and especially not ones you buy in advance.
Trying to understand Transport for London fares can be somewhat of an arcane art, especially since they "simplified" the webpage about them. But you can get the information you need, and it is all at http://www.tfl.gov.uk/cdn/static/cms/documents/tube-dlr-lo-adult-fares-jan-2014.pdf
A Travelcard for zones 1+2 where the tourist attractions are is £9. It is 10 pence less if you only use it after 9:30 in the morning, and costs the same no matter if you take 1 or 100 qualifying journeys. If you go to France it will sit in your pocket still costing for that day regardless.
If you use an Oyster you do have to put down a deposit, but you get that back at the end unless you have fallen into the pothole of a "Visitor Oyster" which has a smaller (£3 vs. £5) deposit but on a Visitor one is non refundable and any claim for unused Pay as you Go money will only be mailed to you in a British cheque after you mail in to them, whereas you can collect both the refund of Pay as you Go money and deposit in cash from a normal Oyster after you have finished using it.
With an Oyster you only spend what you use in rides. If you don't do much one day it doesn't charge as much. It maxes out at £7.00 in zones 1+2 if you only travel after 9:30, at £8.40 if you start earlier. And if you want to go to the airport it will figure that out and you don't need any other ticket (if you have sufficient cash on the card).
All in all, I would suggest a Pay as you Go Oyster.
Happy travels