I'm not a uni student but I am going to apply to Oxford this year, our school did C1C2S1 and FP1D1M1, will do M2M3 this year. The preferred choice for Oxford is at least three modules for mechanics if you're doing the full further maths AS, the D1 module is slightly frowned upon but it won't make any difference just at AS providing you also do mechanics. S1 is sort of in between.