Actually 50% of top lawyers do a different subject first like history or maths etc so they are rich enough to have that extra year of studying. Oxbridge are not in London. A lot of rich people already live near London so would rather go somewhere other than London for the university experience. If you look at destinations of leavers on the websites of the top private schools you will see a lot of people going to US universities for example and obviously Oxbridge, Durham, Bristol etc.