Erm.. why wont it allow me?
Hmmm...how to explain touch to you?
Its like using different joysticks to play computer games. Some joysticks are with knobs and buttons more sensitive than others. You get used to a certain joystick and can't play a game as well as you would have if you use another.
Its like using different poles to play pool. Pool players often have their favourite sticks. They are comfortable with the weight and the smoothness of theirs favs to play a better game.
So, taking piano exam...you will want to practice on the closest match to the one you're taking exam on. In this case, classical piano exams use the accoustic piano. The sensitivity of the keys are different from the digital piano. Your hands need to get used to the sensitivity level of your tool to pass the exam.
And playing the piano....is a highly mechanical activity which relies on muscle memory. That means, when you practice, your hands automatically memorise how to play the song. That includes where to reach for the notes without looking, how far the space between the keys are and how hard and how soft to whack.
Therefore, practicing on a P85 to pass an exam tested on an accoustic piano will be very challenging. Coz your hands will play as they will on a digital and due to the different reponse the accoustic piano gives (and the accidental slips of fingers because the feel of the key surface also different), the piece which you play beautifully on P85 might sound crap on the accoustic.
Imagine, a digital piano and an accoustic piano both have weighted keys already have different response to your playing, what more to say about non-weighted keys? Non-weighted keys are smaller than weighted keys. All the distance between the keys which is memorised while you practice on a non weighted keyboard will cause a disaster if you go to an exam on an accoustic.
In order to REALLY save money in the long run, I suggest you decide if you want to take piano exams later on. If you do, go for an accoustic right away. If you don't, then your choices will be more.