When I last travelled to London, with kids, I was researching how to get an Oyster card. I'd used one before but borrowed from relatives in London, that wasn't going to work this time.

So I assumed I'd need a "tourist" Oyster as I wasn't a resident. But when I looked in to it as a tourist you had to book well in advance so they can post a card to you - whilst if you ignore the "tourist" offering you can just pick up the card at your arrival station (Victoria for me, very easy, was from a machine IIRC). Also the charges seemed higher for tourists somehow and there seemed to be difficulties giving the card back and getting a refund - I could see no reason to have a tourist Oyster.

One of the kids travelled free, with me, as it happened which was great. The only problem is that the automated turnstiles wouldn't let us through separately without us paying twice (for which we'd need a different Oyster) and I got "munched" on a couple of occasions one of which gave me a very sore back, it was quite vicious.

Traveller beware!