"Outside of the U.K., people assume that it's just a bit of a joke -- five pounds here, five pounds there," Rupert Adams of William Hill said. "It's actually really genuinely quite big business."
So far, most people are betting that the queen will wear a blue hat and that Elton John will sing the first song. Other favorites for the first song include Ed Sheeran, Coldplay, Joss Stone and James Blunt.
Storefronts are also decked out with all kinds of royal wedding souvenirs like tattoos, bookmarks, spoons and keychains.