My siblings went to Oxbridge and I did not try - our in NE England school until my younger sibling got in had never had anyone there. It did not hold me back but I probably could have been Oxbridge material really and was top of my year in law etc won prizes and that kind of thing.
My 5 children could all have tried Oxbridge but having assessed the effort involved coupled with what they thought were for them a low chance of getting in they did not bother. In my twins' year at private school 100% of those who tried Oxbridge failed which is not surprising as it not as academic a school as my other 3 children's schools.
I think top law firms recruit about 7 or 8% of their trainee solicitors from each of Oxbridge and Cambridge so about 15% in all. Durham is about 5% and Bristol similar. (3 of my children all lawyers or nearly so went to Bristol and one lawyer one to Nottingham (4.5%)).
If Oxbridge starts letting in people who are not suitable to advise law firm and private equity etc etc clients then companies will hire from elsewhere. I do not think that is the case so far. I did an experiment earlier this week in looking at those people in one of the groups my sons are in on their final law course to look at those who have a training contract at a law firm once they finish and pass the course. I wanted to see where they were coming from in terms of university attended, type of school etc. This is very anecdotal . They have 4 people in their group on a corporate subject with contracts from a big US law firm. It is based on what the students write about themselves on their linkedin profiles. They all have firsts or 2/1.
There are different groups of only biggest firm trainees however and I bet those have more Oxbridge than my little list below.
Ms X went to a boarding school and then Bristol
Ms Xi, very good London academic state school, then Nottingham
Ms Y, very interesting background in Africa, then a UK university, then UCL
Then 3 from a mid ranking UK London firm
Ms Xi ii, degree in China, BSc Manchester
Ms Z, state grammar, Bristol, 2 years as paralegal
Ms Y, well known girls' boarding school, Nottingham U
Mr A - another US law firm, well known London private school, Oxford,
Ms B - mid ranking London firm, private school near London and then a BSc 1st
Then different London mid ranking firm
Ms C, Liverpool - not sure of school
Mr D France schooling, then US degree
Fairly large firm
Ms Ei, first degree abroad where she is from and second degree then Warwick LLB
Ms F state grammar then Durham and mentions various special programmes for the disadvantaged.
Middle ranking firms London:
Ms G, private school in Oxford then Leeds, then Bristol
Mr H, N Ireland, then degree abroad and another at home
Ms G, leading girls' academic boarding school (seems to have A* in everything ever, top 1st etc) - I didn't note down the university I think it was Oxbridge for sure
Ms H boarding school then Edinburgh
Then my twins - day private school London and then Bristol. I was actually quite surprised how many of the above were from the 20% who go to fee paying schools at age 16 given the aim to get people into these jobs who are not from that. I suppose as the law exams and the tests are pretty tough and objective, it can be a bit late to pass all the tests and assessments at 20 if you have years of disadvantage behind you.
Moral of the above tale though may remain if you are a mother work full time and pay school fees - it pays off.