agreed, toynbee is right that only from a very narrow viewpoint is the sorting of the best from the better really a key issue in education, BUT...this is an oxbridge forum. Most of the people in it are likely to be in the 3A 'top 3.5%' bracket she dismisses, so of course these are issues we care about.
As for your second paragraph:
the problem you are ignoring is the question of 'merit', implied by the advocacy of meritocracy. On what basis does a person 'deserve' a place at university over another? If ability does not count when that ability was acquired through finiancial advantage (and it is not self-evident that this should be the case), then how is this distinction to be made?
How does 'paying more attention' to the fact that some private schools provide a better education than the state sector help?