The only limits on instruments are when using the plastic ones - they are only allowable up to a certain Grade level (I think it is 3, but might be 5 - haven't got the relevant bit of paper in front of me at the moment).
You can use borrowed instruments and borrowed music - but not photocopies.
At our school last week an oboist accidentally dropped her instrument wrecking a few keys so had to borrow the instrument of another candidate so she could carry on (luckily there was another oboist candidate with a decent instrument!).
With Rockschool it's compulsory to use the drum kit provided until Grade 6 and above where you can bring in your own kit.