Just got into London two days ago, went to Euston Rail station to buy my 7-day paper travel card.
For those of you who previously asked, you may not purchase a Travel card planning to start its use several days after purchase. The card is valid from the time of purchase, and the agent I dealt with was fiercely adament about that.
I know someone said if one agent would not do it to go to another, but then you have to get back on the end of a long queue and hope you don't get the same agent or the same answer from another agent.
We have already used our 2-4-1's at the Tower of London and The Globe. Even when I offered to show it, no one was interested in seeing our travel card. They simply gave us the discount, and I have to say, it does add up to a nice savings.