We are making our first trip to England next month. We will arrive at Heathrow, but are initially passing through London, taking the tube to Victoria Station to travel by train to our first destination, then a week later returning via Victoria, and taking the tube again to King's Cross for another train to our second destination, and, finally, three days later, returning to London for two nights before we head back to Heathrow and home.
Long preamble, I know, but here come the questions: Is an oyster card time-limited, i.e., if we get them when we arrive, will they still be usable a week and a half later? Also, should we buy visitor oyster cards before we leave? Can they be purchased when we arrive at Heathrow? And what's the difference between a regular oyster card and a visitor card? Thanks for your help.