Two points,
1. If you're concerned about 'social mix' (eg rahs or whatever) the rule of thumb is the cheaper the halls, the broader the social mix, ie when I was at Lupton there were loads of people from state schools, northerners, Welsh, Scots etc and it was pretty down to earth and chilled out. At the more expensive halls, by definition, you will get more of the students from rich backgrounds. Most people are still going to be alright, but the spoiled princesses and gap-yah Ruperts who you will find at every uni, will be at the more expensive halls, they won't be at the likes of Lupton.
2. Regarding self-catering or catering, I did self catering. I can't comment on the quality of the food (although if you're on campus and being fed by the Refectory it will be pretty good). I am convinced now that self-catering is the way to go, you get more freedom and a few friends I had who lived in catered halls found it a bit of a bind having to be in at certain times to eat cooked meals. I suppose there are some totally dependent guys who have never boiled an egg who will just be too lazy to feed themselves, but in general I think girls prefer self-catering, because girls social activities involve more around food, they go off buying ingredients and cooking together, its the sort of social thing to do on a Sunday, and the ones who don't have a kitchen miss out on that.