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.

Interestingly, some colleges tacitly subdivide these groups further.

Christ's Cambridge famously used to hand out "2 E" offers to those in the first group who they know would put the effort in anyway, but "3/4 A" offers to those in the first group they thought might just coast with a 2E offer.

A "2E" offer was certainly a mark of prestige.

(You get your offer in around December time, but sit your A levels in the summer).