I did my undergrad in Cambridge and my masters in Oxford and was fortunate enough to do well in both (I only mention this because it's relevant to what follows). Michael, I remember very well the feeling of starting out and being worried that everyone else is going to be so much better, that you're not good enough to be there, etc. This is natural and it will pass as you get stuck into the course. You have a fantastic opportunity going to Oxford to do law and you shouldn't let your A level results make you feel insecure when you get there, for one simple reason:
The kind of thinking and learning you'll be doing in Oxford is fundamentally different from the kind of learning involved in your A levels.
There's a much bigger emphasis on independent learning and original thought, and the "syllabus" is just a guide rather than a complete list of all the things you need to memorise. Of course A levels are relevant to admissions and so help you get your foot in the door. But if you look at the group of people who get in, you definitely can't predict their degree results (or, more importantly, their legal reasoning abilities) just by looking at their A level scores. I knew people with pretty much 100% UMS in all their A levels who were average lawyers (not a bad thing). I also knew people who far exceeded their A level achievements to become absolutely stellar lawyers. Put briefly, A levels and Oxford law develop different skills and test different things.
I would be so so happy if even one person would read this and throw themselves into their university course without worrying about UMS scores any more. What's exciting (and a bit scary) is that you'll be judged on how good a lawyer you make yourself, not on your past. My advice: get stuck in and don't look back!