Why did you apply if you don't like the collegiate system that they use?
If you don't like the University, then don't go. You don't want to spend 3/4 years of your life at a University if you won't enjoy it, you don't just go for the degree, you also go for the experience, meeting new people etc.
As for career prospects, yes, it is true that employers will prefer Ox-Bridge graduates to others, but you need to realise that you'll still have good career prospects depending on what area you want to go into (not everybody else applying for the job will be an Ox-Bridge graduate!) lots of successful people have been educated elsewhere. It'll come down to what degree you get, most employers will prefer a first from say, Manchester, than a 2:1 or 2:2 from Oxford.
Your other Unis are very good ones too, you'll have no problem getting a job if you get a good degree from any of them.