Johnny Depp isn't English in real life. However,in the movie as Willy Wonka,he is supposed to be English because Willy Wonka is English. The movie was filmed in England so you would assume that all characters would be English and that the money would be a pound, not an American dollar. So I agree that it is a slip up.
Chelsea I know Johnny Depp is not English but if you read the book it says that Charlie found money on the floor. So how could Depp have just dropped his money Freddie Highmore find it whilst being watched by a camera? Also for the person that said about the tourist, why would the shopkeeper accept the money if it was American and Freddie Highmore has a very British accent.