zaeem1, I get what you mean. DC were at academic London private schools, and although white Caucasian, their school friends came from all sorts of backgrounds. DS was in his element at LSE where his friendship group was a complete mix of British born kids from English state schools, more recent East European migrants, ethnic Asians (South, East and SE) from Britain, Asians, third countries or via British boarding schools, as well as a sprinkling of Aussies, North and South Americans, Australians, Africans, Germans, Italians and Scandinavians. The French oddly, were the ones who seemed to keep most to themselves.
DD is at Bristol. She had wanted something that was "London, but not London". Her first year was quite problematic. She was surprised how white and relatively mono-cultural Bristol was. I think the issue is not really the "rah" factor but how sheltered many of the other students seemed to be. Then away from home or boarding school for the first time, they arrived determined to sample "the University experience", though perhaps without the experience of mixing with, and being sensitive to those from other backgrounds. DD was just not interested in clubbing, and had been used to a culture of work hard/play hard and took a while to find her people. Oddly her people have turned out to be outdoorsy types from rural areas, often from north of Watford, or Wales and N.Ireland. Two friends who went to Exeter, similarly, kept well away from the private school sets, and found their people through shared interests.
People on MN can be very rude. There was a poster with a similar posting style to MyOMy who effectively suggested my DD was a Billy-no-mates because she did not share the clubbing and drugging interests of her first year flatmates. (The peer pressure was immense, and others succumbed, but I suspect London kids have earlier exposure to a party lifestyle so have more confidence in not going along with it.)
We know a couple of former London private school pupils who found the "rah" element at Bristol and Exeter so difficult that they chose to transfer/restart in London. And indeed DD is taking a year out of her medical degree to intercalate in London. Since making that decision she has decided she loves the west country and is now likely to apply for training positions outside, but within striking distance, of London. She loves Bristol but not the University.
So of all your choices, I would pick Durham. It is the first choice of many from cities in the North, so will have a greater natural diversity. But with Geography courses can vary a lot. The LSE one is very urban, so perfect if he wants to head towards a career in urban regeneration or town planning, but not so good if his primary interest is geology. They have some impressive names on the staff. LSE suits those who are quite course orientated. DS was very happy there (econometrics) and his friends were based on a shared interests, whether course or societies. LSE is a big change from school, bigger than Oxbridge in many ways, so it did not matter that he has only moved about a mile up the road. LSE is just too diverse for there to be an obvious private/public school divide. Everyone comes from somewhere and you just get on with it. That said it is a slight pity that DS has not lived anywhere in the UK other than London.