Hi RHN, I think there are lots of really excellent universities in the country, and oxford and Cambridge are just two of them. generally people need to get good a-levels and to answer questions in an interview well.
More and more though the universities are trying to find people with good potential, even if they maybe haven’t got the very best results in their a-levels, and this is important because sometimes people don’t necessarily get the results they are capable of when they are relatively young at school.
If you are interested in going to a university like oxford or cambridge, you could contact one of the professors in a subject that you would like to study and ask if you could meet and have a chat about what it is like to study there and how to get in. This is what I did when I was deciding where to study and what to study.