Take a Mathsaurus online courses to help you prepare for Maths Challenges or to start A-level maths. There are free courses for the Primary Maths Challenge (ages 9-11), Junior Maths Challenge, Senior Maths Challenge and Junior Kangaroo and Olympiad. All 70+ teaching videos in the course Get Ready for A-level Maths are free too so take a look and sign up now!
Information and past papers for the UKMT Junior Maths Challenge, Intermediate Maths Challenge, Senior Maths Challenge, Olympiad and other competitions can be found at https://www.ukmt.org.uk/ . There are past papers and solutions for recent papers there, as well as an archive of individual questions and solutions going further back. I have compiled below grade boundaries for each challenge: