How exactly? the way it's managed at the moment is that the admissions tutors keep an eye on what different schools are doing in terms of prepping their students and keep that in mind when assessing them. Which is exactly what causes the 'bias'. I am significantly more impressed by someone who manages to get good grades with little help than someone who manages to get excellent grades with the most effective teaching of the test that money can buy (having gone to an independent school, I was one of those being 'discriminated' against, but I can well understand that many of my classmates, who primarily good at cramming for the exam and not understanding the material, got very good grades but would not do nearly as well at university. Heck, I got in and struggled way more than I had done before, and more so than many in my year who had worse grades in less good schools)

(I would, in general, be in favor of fixing GCSEs and A-levels. They have persistently moved in a direction that rewards memorisation of particular keywords, something which especially rewards teaching the test, as well as getting easier and especially less good at discriminating the top end of ability accurately. But it's still not going to be enough to remove this difference)