Bubbles how did she know which first year options would be unpopular?
This is probably the big difference. DS had two or three compulsory courses in his first year reading economics at LSE, and he took an option from the maths department which was compulsory for maths students. So up to 350 in lectures and quite a lot in classes. However there were a lot of other things going on, like a voluntary lecture series involving PhD students giving papers, or the first years themselves giving presentations. He and his friends were also supported in doing some research which was shortlisted for some US UG conference. If they had won the LSE would have paid for them to go over. Plus Economics Society stuff, eg a trip to Cambridge to meet a Nobel Laureate, and a lot of public lectures at LSE and elsewhere. In his second year he was allowed to sit at the back in peer review meetings for PhD students, and in his third year he did some voluntary teaching on a SU organised "quants for quals" course. (Statistics for social scientists.) I have heard of others getting similar opportunities and access to world leading academics at both UCL and Imperial. (The research one was funny. A group of earnest first years went to see a Professor, who kindly opened his address book and they were soon hurtling all over the city and civil service interviewing some very senior people.)
So the big "but". It helps to be really absorbed in your subject, so fully able to take advantage of being in a top research institution which is 50% post graduate, and hugely international. And you have to have the confidence to put yourself forward. DS is now studying for a PhD in the US, and his technical preparation is as good as anyone's whilst the social networks formed at LSE proved valuable when he first arrived. I think there are similar advantages for those who arrive at a London University with clear career ambitions. Subject by subject I don't think there is much difference if any in career opportunities and starting salaries between Oxbridge and the more respected London Universities. The opportunities are there if you look for them, but you will not get the nurturing you might get at an Oxbridge college. DS had friends who would have been happier elsewhere.
Plus terms are longer. You still have to work very hard, but at least you have a bit more time to cover the ground.
Pease, we know a few who went to Oxbridge because they did not want to let parents down. DS and his closest friend were able to see immediate advantage in their Cambridge rejections as the courses at LSE and Imperial were preferable. (His friend similarly had amazing opportunities at Imperial, mainly gained through involvement in a subject related society.) One friend, who had preferred a course in London, dropped out of Oxford because there were so few teaching hours and the tutor had little interest in students.
I simply wanted to be in a big city after boarding school. One of DS' friends was the only ethnic minority in his Welsh Village and was attracted by London's diversity. A couple of DS' friends were first/second generation immigrant with no tradition of going away to University. DD had she been offered a place in London saw advantage in keeping up with her existing sports. Whilst others, often after a gap year, decided on London over other RG because they felt too old for the whole freshers/drinking culture stuff.
Our DC are very lucky to have such good choices.