1. You don’t have to study Law. Study the subject that you’re passionate about. Only you can know what that is. If happens to be Law then study that.
2. I applied to oxford with GCSEs lower than the majority for law and got rejected. I applied to Oxbridge because I had nothing to lose, as do you. Some people have gotten into law at oxford with no A*s at GCSEs. You could be one of the lucky ones that gets in so don’t let your GCSEs deter you.
3. Check out the courses for both before you make a decision on which one to apply to, as well as their application processes. I personally applied to oxford because I liked it more and didn’t want to sit a test on the same day as I have multiple interviews (which is the case for Cambridge).
4. Do sit the LNAT as a lot of really good unis for Law require it, but also apply to ones that don’t in case it doesn’t go as well as you wish. The LNAT is all about practice - do plenty of papers in the run up to the real thing.
5. Short answer: no. Just make sure you go to a top uni and you’ll be fine.