Regarding the Visitor Oyster Cards, I didn't want to dump on someone else's thread or misdirect it. I've not kept up with this in the last couple years like I use to, but it once was the prevailing opinion not to bother with Visitor Oyster Cards. They did not offer any advantage over regular Oyster Cards, the purchaser was paying the £5 card fee plus shipping and discounts & travel cards could not be applied. Plus they could only be purchased through the "Visit Britain Shop".
Now they are available directly from TFL, Youth Visitor discounts can be applied, and other than contactless/mobile payments being just being too easy and Oyster Cards are still simple, what's the disadvantage for Visitor Oyster Cards?