What are the best universities in the UK for studying English Literature or Political Science?

I'm a sophomore in high school in the US and I'm interested in studying at a university in the UK. I've gotten letters from the University of Cardiff, the University of Portsmouth, and York St. John University. Being unknowledgeable of universities outside of the US, I don't know if these are reputable universities or not. I want to go to a well-known school. I have a 3.6 GPA and am involved with many different extracurricular activities, as well as having had poetry published before. Because I don't have stellar grades, I doubt I'd be accepted to a university like Oxford or Cambridge, but what are some other well-known universities that would accept me, given that my grades will improve? I'm interested in studying English Literature or Political Science.