I'd recommend using pay-as-you-go on a contactless card or Oyster card over buying a travelcard, unless you'll be doing a lot of travelling on the Underground or National Rail ("normal" trains). It'll probably be either cheaper or the same price, and you don't need to worry about expiry.

If you have a contactless card there's nothing to arrange, just tap on the reader. If you want an Oyster card you can buy one in advance.