It sounds like she's nervous. She's certainly showing all the right signs of being ready but if she is refusing to use the toilet/potty at all the question is if forcing her to would be worse for her than waiting until she's ready.
Have you noticed previously in any circumstances that she's had trouble dealing with transitions due to being a perfectionist and wanting to do it right straight away?
Personally I would wait until she's ready to sit on the potty/toilet. A couple of ways to encourage her would be to take her out to choose her own new potty or toilet seat and try and encourage her to sit on it with her nappy on to get used to it.