Do you reckon that in the 4th year you could graduate with Part III Mathematics and Part III Astrophysics, since there is so much overlap between them I.e. if I followed the Natural Science route (Maths, Chem, Physics, CS -> Maths, Physics, Advanced Physics -> Astrophysics (II & III)) but attended a few more lecture courses in Part III Maths (probably the more financial courses) as well as the prerequisite astrophysics ones, could you graduate with CASM and Part III Astrophysics?
Maybe I am underestimating the workload...
In fact, maybe I ought to think about trying to get in first!