My son's in second year of Cambridge doing engineering. Thriving. He is also I was very worried as he is a quiet, v studious boy who is verging on antisocial. We are also not "posh" ha ha! I also suspect he may be on the spectrum but we've never sought a diagnosis as it has never affected him - he has always had friends and been a happy chap. He was never worried about moving away from home, and meeting new people although small talk doesn't come naturally to him. He's just very nerdy, IYSWIM? However he is very confident in his own academic ability and also when he was in 6th form did lots of projects in his own time, as well as entering the usual olympiad type competitions. He was top of the school too, in a superselective state grammar. Estimated to get 4 A stars which he got, and he was never concerned that he wouldn't get that. I don't say this to brag, just to illustrate that perhaps the people who thrive there are those who are naturally super clever in their subject, self-starters and HIGHLY motivated.
He does reckons he sometimes does up to 50 hours of work a week - he ALWAYS seems to be working or revising. A "free" hour on his timetable is not free at all - he'll always have work to do or revision to do. He reckons some people do less than that (certainly they do in other subjects - engineering is one of the most difficult Cambridge subjects) but he always sets himself the challenge to come top in his year, or at least in the top 10%. I think he thrives on the challenge and seeing what he can achieve - engineering at that level is pretty out there in terms of academic difficulty.
I agree it's a different world if you don't know anything about the process, which I didn't. DH never even went to uni, I dropped out my first one and just ended up going to an ex poly getting a bit of a useless degree! But to be honest, it has all came from DS. He said he was going to apply for Cambridge as he thought he was good enough, did loads of research himself on the application process before telling us, spoke to teachers. So by the time he announced it to us he was pretty confident that he had as good a chance as anyone. Then it was DH and I who had to do all the learning of the application process!
As others have said, Oxbridge is full of uber nerds/people on the spectrum. At the interview they won't care about hobbies, being captain of sports teams, part time job etc like other unis might, it will be VERY academic, solely looking at ability and whether you are teachable according to their style, as well as motivation. You do have to be very dedicated and determined about the whole thing. And resilient. It's a pretty rigorous application process with multiple steps. Many people get rejected without even being invited for interview or to do the ENGAA test (look that up if you haven't heard of it). So he'd need to be prepared for knockbacks at multiple stages of the process, and just see how far he can get. As DS said to me at the time, "SOMEONE has to get those places, why not me?"
Having said all that about how demanding it all is, I think Cambridge has REALLY suited him, and we felt that when we went to look round in the summer holidays after end of Year 12. I think he saw himself there straight away. He has also met some nice friends and has a LOVELY serious girlfriend, they seem very well matched. That's the thing - there are lots of other like-minded people just like them.
I've asked him what he thinks of it all now he's done 2 years there and he said he definitely would have had a more free time at other unis that his friends are at, so would have been able to have more down time to have fun. He still does have fun at Cambridge but it's definitely more studying than partying.
Feel free to PM me if you want to know anything else. Oh, one thing. Pretty much everyone there doing engineering had all A stars at A-level. Nearly all had further maths, I think. He said he thinks it is different for some other less indemand courses eg some humantities.