WRT the astrophysics question, I doubt that would be possible for a medic in their Part II - my understanding is that it is quite a competitive part of the physics course to get on, so without having done Natsci physics (and probably Maths) IA and IB, you probably wouldn't get on it. In general you can only do the biological options of the natsci tripos - we just don't cover enough maths etc to go into a physnatsci part II. The people who do weird stuff like languages or music generally do IA.
As for assessment at clinical school, you have OSCEs and written papers at the end of 4th year (although a little earlier than usual, about April time). You then have a few end-of-placement tests in 5th year, both OSCE and written, and the BIG pathology written exams, paeds and O&G OSCEs at the end of 5th year. Then finals are both written and OSCE in June of 6th year. It sounds like a lot but actually there's quite a bit of time in between all these, you do not have to be revising all the time!