nooka, hatwoman, you're right: Oxbridge undergrads are supervised more closely than in many universities and the requirements made of them are, for the most part, greater. Therefore most of them probably work harder than many students at other universities. I've taught elsewhere and while there are plenty of students in other universities who choose to work hard, they aren't under the same pressure so to do, and the requirements (e.g. how many essays they are supposed to hand in, presentations to give, etc.) are not in the same league.
hatwoman you'll be pleased to know that we have moved on (a bit...) from depending entirely on final exams taken in a very short space of time! So there is more coursework (e.g. extended essays, thesis) required in most subjects. It's still very heavily exam-based compared with many other universities though.
On your questions: No, I don't think that failure to surmount the very serious obstacles of unsupportive (for whatever reason) schools or parents necessarily indicates lack of suitability - certainly it doesn't indicate lack of academic potential. There is a real barrier here, based not least around (mis)perceptions about Oxbridge, and it is emphatically not a good way to filter out people who are not suitable for a place.
There is a limit to what Oxbridge itself can do to challenge these obstacles: we cannot take people who do not apply. But we do a lot: there are outreach programmes to try to get people to think about applying (what you said about your dh's school legally is very sad and rings all too true), and to try to get them to see what opportunities there are. We work alongside organisations like the wonderful Sutton Trust to give people in weaker schools the opportunity to spend time in an Oxbridge environment on summer schools etc. And we try very hard, when people arrive from different educational backgrounds, to level the playing field in the first year (e.g. help people who aren't used to writing essays, something on which the arts subjects rely heavily in Oxbridge, to get used to doing it).
Why do we blame this wholly on Oxbridge rather than in part on the schools? Ummm...because it sells more newspapers? And, less cynically, because no matter which way you look at it, our state schools are struggling desperately with inadequate funding while Oxbridge is relatively speaking rolling in cash (yes, I am aware of the nuances here)?
Finally, good god, I haven't been to a nightclub in about fifteen years