Cambridge and Oxford are known in academic circles to have many students with mental health challenges. Most students there were in the very top if their HS graduating class, yet half will be in the bottom half of the university graduating class.

Further, UK degrees usually are sorted into "1st class" (best), "2nd class (top half) -- aka 2:1, or the dreaded 2:2. Often there is a % cap on the 1st class degrees. The difference in achievement between a bottom 1st class and a top 2:1 can be small indeed.