Should I get a BSc at a top-ranked university or a MComp at a lower-ranked university?
If you can attend a top university, do so.  Having other excellent students around can provide a much better learning environment--to the point that in three years at a top place you may learn as much as in four years at a more mediocre institution.  If you do well, you can always do another one or two years of graduate study after your bachelor's degree.