I haven’t checked recently. It was 10-12% in my day. It’s sometimes argued that a national standard should apply for firsts. If this is the case, and if Oxbridge really do reach the highest academic standards, then you’d expect them to have more firsts.
They used not to separate the seconds into 2(i) and 2(ii), not least because the separation point would be close to the peak of the distribution where a small error in assessment would lead to the maximum number of people ending up on the wrong side of the line. In the case of students with seconds applying for post-grad funding, which was sometimes awarded only to those with 2(i)s, the funding bodies used to contact the university and ask the examiners whether each student was 2(i) standard or 2(ii). One of the tutors in my college used simply to write back in every case saying “If this student had gone to a university which separates the seconds then he/she would have got a 2(i)”.