You need to report her if she really threatened to not send off your application. Report her to the head of the college, and say if they don't act, you will report to the LA and the papers. That is awful behaviour. It is not her place to force you to go somewhere, especially for a course that isn't what you want to do.
It is your life not hers, and there are no guarentees you would get into Cambridge, so it could just as easily be a wasted spot on your application (never mind a waste of three years if the course doesn't give you QTS).
Yes, there are negatives to being a primary school teacher, and it can be hard to find jobs in the most desirable schools and some parts of the country. However, if it's your passion (and it clearly is), then you should go for it.
If you need to, however, you could be sneaky and get around her. You don't have to add all your ucas choices at once, so just put two on, get her to send the application (by lying and saying you'll add Cambridge later) and then add your other three choices.