Heavily doubt anything like that happens, the media would be on any hints of this practice happening like a pack of killer bees. These anecdotes of people getting into Oxford through bribery seem pretty insubstantial to be honest. Oxford's not exactly doing badly for finances so I doubt it would take risks which would defame it so badly. As it is both Oxford and Cambridge already receive so much bad press from accusations of elitism, racism etc.
I reckon that if there is corruption in our higher education system it exists in the tier of unis just under Oxford (LSE, Imperial, UCL etc.). Obviously they wouldn't be able to attract as many bribes as Oxford/Cambridge could but they could still attract good money. And also there's more incentive for these unis to be corrupt because they're less well-off and don't have quite as big a reputation to damage. Indeed there was that whole fiasco with LSE and Colonel Gadaffi (can't remember the actual facts from that but serious accusations of accepting bribes were levied against LSE).