From my experience with UMEP (and contact with other UMEP students), you generally need at least 35 SS for the subject you aim to do extension for (if you completed the specific VCE subject in year 11), and/or averaging at least B/B+ across all your subjects you did in Year 11 (generally semester 2, but semester 1 can also be considered if you send a report of semester 1).
So, to answer your question, the prerequisites are just a guideline on what kind of student the program is aimed at.
Take note, that there is no competition when they choose who partakes in the program, as it's only decided by your grades. Of course, you need to gain permission from a teacher to take UMEP, since there is a teacher/parent form that needs to be signed alongside your application.
Here's a tip to convince your teachers/school if you really want to do the program; you can always quit even if you are partaking in UMEP that year, since there is a time frame where you can choose to opt out (there should be a time framed provided on the unimelb website, but it's generally until May where you can quit), if you so wish. So, if you are not able to cope with the workload from your extension subject, you can quit. No penalties applied.
Good luck for the future, if you choose to do UMEP.