Math entrance exams (MAT/STEP)

STEP is quite a bit harder but with preparation is definitely doable. MAT is slightly easier but it is more different to A-Level in structure.

What kind of scores do oxbridge normally require

In terms of raw marks, they change widely each year. MAT scores are considered contextually so if you go to a particularly high-performing school then you may need to do better than someone with significant widening participation flags. STEP is graded as one of S,1,2,3,U (S being the best and U being the worst) and there are two papers. The standard offer is a 1 in both papers though there is some leniency for missed offers and they may also set higher requirements for certain students, e.g. post-qual applicants.