In the education system in my country, the distribution of the top grades (in single subjects) is as follows:
10: 0.5%
9: 2.4%
8: 12.5%
6 and 7 are considered mediocre, with 6 being the lowest pass grade. Oxbridge offers are based on an average of 8 (achieved by the top 4%) with 9 in relevant subjects. However, we study at least 8 (major) subjects and only the top 15%-20% follow pre-university education to begin with. So, in order to get an Oxbridge offer one generally has to be in at least the top 1% of all students.
Does it work? Well, not really. In my country university admission is only conditional on passes, so very few people try to get top grades in the first place. Although, for medicine (and some other courses with limited places) an average of 8 or above will give you unconditional admission. Even if university admission did depend on it, I think it would not work. Although performance in secondary school qualifications is somewhat predictive of higher education performance, I don't think that minor differences (getting a 9 or 10 can sometimes depend on 1 or 2 minor mistakes) are indicative of future performance to the extent that they should be used in admissions. That would be giving the secondary school qualifications way too much credit.