I think Anne is currently a bit more common than Ann, at least in Scotland - maybe because Ann is mainly an English language name, while Anne also exists in various other European languages.
I disagree that "Famous Five" Anne never has an opinion - when you read the books now, it kind of jumps out that Anne is the only one calling George out on her "I'm fab so I'm a boy" shite. Yeah, Anne's "nice" and polite and non-confrontational (so is Dick - how's that for a name that's fallen out of favour?) but she's not a total pushover at all.
The lowest-effort name ever. Ann. Just a syllable. Couldn't even be bothered adding an "e" for flair.
A lot of other names fall into the same category, though - for example Lee, Eve, Fay, May, Mae, Bea, Dee, James, Jay, Kay, Kai, Jan, John, Joy, Rex, Roy, Max, Guy, Ash, Bay, Haf, Hugh. Some people like those kinds of names.