In my opinion (with some experience of university admissions), no-one gets into any university when they would have otherwise not done so on the basis of extra-curricular activities which bear no relevance to their chosen course. For the purpose of university applications, grades (both predicted and achieved) are far more important than anything else, and may be the only factor that is used when deciding to give an applicant a place or not.
The PS is often not read at all, or skimmed over to check there are no extenuating circumstances mentioned on it that have not made it into the reference for whatever reason.
Obviously sometimes the PS is part of the decision making process, and in this case onew important thing is usually to show the applicant has developed an accademic interest in their subject outside of the classroom. A very common (and cheap) way to show this is by reading around the subject- with the books discussed and reflected on, rather than just name dropped. Also, for applicants, if reading around the subject they think they want to study in y12 bores them, it might help them realise it's actually not for them!
Another thing that can be tricky is work experience. Obviously for some courses, this is required. However for non-voccational courses where work experience is tricky to obtain (e.g. psychology) some admissions tutors may ignore the work experience mentioned, as it is usually gained through family contacts, and not an oppourtunity everyone will have.
In my opinion, it is very hard for a PS to make up for poor grades. However, a PS can lose a place that you might otherwise have got- e.g. through clearly showing the applicant has applied to other unrelated courses, or through poor English.