I am the graduate of three Russell Group universities, none of them Oxbridge. My hugely likeable officemate is the graduate of two, and both of them Oxbridge. Our boss as well is the graduate of two institutions, neither of them Russell Group and one an ex-poly.
You can take away from this true but anecdotal and unscientific snapshot what you will. What I like to take from it is that your future isn't set in stone if you graduate from non-stellar universities.