This is slightly mitigated by elite graduate schools selecting good candidates from elsewhere (unlike for academic hiring). The large majority of Oxford DPhil s

This is slightly mitigated by elite graduate schools selecting good candidates from elsewhere (unlike for academic hiring). The large majority of Oxford DPhil students didn't go to Oxford as undergrads; I went to Leeds and most of my cohort was international, for example. The expensive school to Oxford pipeline is definitely a thing, but it's much more prevalent at undergraduate level, at least if we're talking about expensive British schools...