I'm not so sure whether it is worth resitting your A-levels. If you get a 1st in your degree, this might compensate for your A-levels. I would also keep in mind that some law firms do not have A-levels requirements and they look at a candidate's journey as a whole. What you could do is to go to open days, law firm events and insight days to discover the law firms that you would be interested in applying to. Make sure you speak to trainees and members of grad rec to get a sense of the culture of the firm and ask trainees about their journey into law.
From there, I would try to prepare applying to law firms going into your penultimate year by mentioning how talking to these trainees or the events you attended persuaded you to pursue a career with the specific firm or why you chose to pursue a career in commercial law. I remember taking Commercial Law Academy's "Writing successful applications" course, which helped me to structure my answers to questions such as "Why commercial law?", "Why the firm?", and "Why me?". I would really recommend it.