It depends on your interests. I know a guy who read extensively not just on maths topics, but on its uses, it's patterns and basically thought of it as an art. Btw I would advise you not to take the view that many have that maths + (something) requires a lower MAT score (or you can get away with weaker maths ability) compared to the pure maths course.
The person I know applied for 2014 entry to Oxford, had all (12.5) A*s, 4As at AS, and 4A*s at A2,+ came ninth in the world in the MAT.