Not only will it possibly get you reduced grades but it's also something really good to write about when applying to university (or degree apprenticeships if you go for any of them). By doing it you can show an interest and depth of knowledge in a particular field and lots of soft skills such as time management. I don't know what a prePQ is as we didn't do those, but our school did a Cyber EPQ which was basically a course on Cyber security and then a project the same as other EPQ's. So I did that and I did my essay on the impact of quantum computing on encryption. I would say do as much research as you can over the summer hols before Yr 12 if possible and decide on a title, then manage your time well and split your project into sections and you'll find it's not too much.