Not only that, but buying said travelcards (one-day or weekly) is incredibly easy. Almost all stations now have touch-screen, credit card accepting machines that let you buy them (the larger ones do the one-dayers, the small, wall-mounted ones both the one-dayers and the longer ones). You don't need a photocard for the weekly 1-2s any more, either.

LHR T123 ticket office is generally a shambles, and the queues for the ticket counters can be horrendous. However, there's usually practically nobody using the CC-accepting machines.