So, moving away from the point of the post- yes there are many smart & cool CTOs, CEOs & MDs that didn't go to Oxbridge.
But what you learn at uni is proportional to delta^2 in the intelligences of the people around you. If it's harder to get in, the people are smarter, and you learn more. There's less drag, classes move faster. You don't need to be a-CEO-level-handshaker to meet the right people, because there are so many. Oxbridge have obscene resources - classes to print your own chips or 1:2 tuition that the next layer of universities down don't get.
There is a mini-valley-effect going on there, that makes it much more likely for someone to act smart if they've been to an elitist university.
Yes I'm bitter. (Yes they rejected me)