To restate your claim (correct me if I'm wrong): Because the laws of physics are constant, there is only small limited possible variability in the difficulty, knowledge transferred, and comprehension needed to take a physics class between that at an elite university vs. any other university.
The facts: Only 19% of American highschoolers take calculus. This already precludes 80% of students from taking the standard physics classes at my university.
Advanced classical mech. classes for first-years can go much beyond that, delving into lagrangian and hamiltonian formulations, fictitious forces, difficulty relativity problems, etc.
If you are claiming half of your year at high school would have been fine taking a course like this, you went to a very unusual high school in my opinion.
But yeah it would seem of the people who were on the path to apply to Oxford (or any other mathy course at a university) would have been fine if they had gotten in.
I'm extrapolating that the other kids in other subjects were just as qualified.
I mean of course I'm not saying people who hadn't done the prerequites would be fine, just the ones who did and didn't get in would be.
It's not like the laws of physics depend on what college teaches them, right? The content of just about any course is going to be more or less the same. A lot of the books were the same as well, comparing notes with friends who didn't get in.