so it can depend per school and sometimes per subject! of course the ucas deadline is quite early in the year (january usually but earlier (october ish) for oxbridge and medicine etc) so you may not have sat your year 13 mocks before your predicted grades are due. although, do not fear! most teachers know that students can crack under pressure, and i'm sure as long as they know what you are capable of, through note taking, interaction in class, attendance, you'll be okay. i had to persuade my teachers not to base predicted on my year 12 mocks (A*BC) and in the end, they raised my predicted to AAA.
also it's worth remembering that if they don't, and you choose your firm uni through ucas as something low, and then you do gain something higher than they're asking for (so for example, if they predict BBB, the uni wants CCC, and you get AAA or something higher than the uni), i found out the other day that you can apply through clearing for a different uni anyway! you can apply for one with higher entry grades and may find yourself at the one you would've gone to if your teachers had predicted you higher.
hope this helps!