They will talk about this "mock test".
It is not a mock test, depending on how strict ur skl is, it will be used to directly make ur predicted grade for uni and once that's set you've pretty much set the ceiling for how good of a uni you can go.
Work hard. A levels are a lot harder than gcse. Unless you go to a selective skl, most ppl will do badly (much lower than they expect). Your skl will usually put grades in a range to make them look better, dont be fooled. For example my skl said 50% of students got A*/A in physics. They did not mention that there were only 4 ppl in that class and no one got A *, they only got A's.
Get a booklet and put any crap you do in it that you can put on your personal statement when you apply to uni. Then when it comes round to writing personal statement it should be easy.
If you're applying to a really high uni check if there are any additional exams required for them and start practicing.
If you are consistently working then you won't have to work too hard and it should be completely stress free.
Find and apply to work experience early.
You didn't get the chance to do gcses so make sure you know how to revise:
Active recall
Spaced repetition
Lots of questions
Follow spec
Pomodoro