I am surprised that you did not refer to any of the research that provides a justification for Bristol's admission policies. Perhaps the best study to date was carried out by the Higher Education Funding Council for England in 2003. It found that university entrants from state schools and colleges are more likely to get a good degree than similar students, with similar entry qualifications, from independent schools.
The extent of the advantage varies by subject and across universities. But if one believes selection should be based partly on the likely degree result, then these results do seem to support admissions policies such as those used at Bristol.
Institute of Education
University of London