If you're an IB student, I doubt you're very far away from being a posho. What's the problem? You don't like the white ones?
If you're rejected from Oxford, would you not also be an Oxbridge reject? It depends on the course, of course. Some courses at Durham are very far from Oxbridge-tier, so there are by far fewer Oxbridge rejects attending there. PPE is not one, though.
The uni is not that mixed. I had seen stats for nearing 70% white. The students are firmly middle class, offensively so. It really doesn't matter to you; you'll mix with them all the same. You won't escape their type at any of the unis you listed. The people who go to good unis tend to come from decent backgrounds -- just the way the world works, my IB-studying friend.
Also, I would slightly disregard Lounger's comments. He had an awful experience at Exetah; did not enjoy it. He thinks this means no one else could enjoy it and thinks to make hatchet-job complaints which are not even relevant to the reason he did not enjoy it. Exeter has fewer privately educated students than Durham; and the Durham lot are far posher.