My son just finished his 4th year reading Maths & CS. A couple of thoughts for you:
The MAT is a challenging test and all Maths/CS applicants have to take it. All applicants (both Maths & CS) take the same three sections - the first multiple choice section and two others. That means that 60% of the test is high level math. You need to make sure that you are really prepared for that. CS applicants then do two separate sections that focus on logic problems. You need to get really good at those. At interview, my son was given a set the night before and had to show the tutors how to solve them.
Some of the core CS classes are challenging. My son found the exams for functional & imperative programming (1st year) and Algorithms (2nd year) to be quite difficult. He has enjoyed his 3rd/4th year classes and did his thesis on machine learning (training a program to read medical images).
Good luck with it. Happy to answer any questions about the process. Best advice I can give is do past MAT papers as practice.