Hehe of course there is nothing wrong with calling Mel or someone posh cause it's just a synonym to stylish. But I believe that "classy" beat this word somehow ;) I agree this is not quite her style but she looks so ladylike here.
Hmm... as far as I know, posh isn't an ofensive word, or is it (and I'm being honest here)? I just meant she doesn't look natural on the outfit. That's not really her style.