To add to this: Oxford and Cambridge always get the smartest in the country, and they don't struggle to identify them. From what I know of the admissions process, there's three fairly obvious groups of applicants to oxbridge: people who are obviously ludicrously smart and driven and will not struggle at all with the course, who are obvious accepts but are not nearly enough to fill a year group; those who are obviously not cut out for it and are rejected outright, and those who are probably going to be OK but it's not a slam-dunk. The main challenge for them is picking those with the best chances out of the third group.