The focus at the top universities is on interview performance and performance in the various admissions tests (STEP, MAT, TMUA). Most applicants will mention things like the Senior Maths Challenge and maths books. There are popular texts like ones written by Simon Singh, maybe basic books on number theory or group theory as well.
In your personal statement you should make it clear that you understand that university maths is about understanding rather than doing calculations, though of course you do calculations in the process.