hi im in y13 rn applying to oxford and about 90% of law applicants (so far lol) are being rejected before interview - oxford holds large weighting on gcses and expects a gcse profile of 8 or more 8s and 9s so i suggest that maybe you perhaps consider cambridge
Cambridge is equally competitive and you would need to make up for your GCSEs elsewhere (I shall explain how shortly), but because they interview around 75% (they give each candidate 1 or 2 interviews whereas Oxford applicants tend to get 3+), the probability of you getting selected and thus being able to prove yourself at the admissions test and interview is higher.
If you're still thinking of applying to high end law schools, I would firstly suggest that if you are not sitting AS exams in year 12 that you should probably apply on a gap year once you have your results. If someone is predicted A*s and As with weak GCSEs, then the university may doubt that you achieving these grades is actually feasible. Instead, show them your results when you have them in hand, which will work to your advantage because a) it will remove any doubt that you won't be capable of meeting the grade requirements and b) demonstrates an academic upwards trajectory, which is seen as a positive.
As for how you can make your application better, read law related books and engage in law supercurriculars. This is essential as not only will it demonstrate an interest in your subject to improve your PS, but it will also inform your legal reasoning and, in turn, make you perform better at interview.
Finally, this may sound harsh but don't get your hopes up too much. Cambridge will still look at your GCSEs and may wonder what went wrong. You will be competing against people with straight A*s. I think if you do everything right from this point on you have a good shot, but as someone who's currently in the Cambridge law application round, I'm not even sure of my chances, even as someone with A*A*A at A-level and all A*s at GCSE. Oxbridge is cutthroat.
I go to a state college and have never been in private school. I’m not 100% sure about statistics but my college typically gets students who get good a level grades (not sure about GCSEs) who go on to Cambridge, not all though. It was one of the top 10 colleges in the country a couple years ago but I don’t know if oxford takes that into consideration. For my GCSE grades, my grades would be seen as average-high from my secondary school compared to the rest of my year.