I’m going to be in the uk for a while and have been told that I should get an oyster card. I’ve seen I can purchase these over the internet through the visit Britain website. Does it make any difference if I buy on in advance, or should I wait til I’m in the UK? I assume you can buy them from train stations and such? I leave on May 11 so it would be cutting it fine with postage, although I can get express shipping which would mean I would have it within 2 business days.