If you do natural sciences, you say when you apply whether you want to specialise in either Biological or Physical NatSci.
For example, I've applied for Physical NatSci and if I get in, my degree will look like this:
1st year: Maths, Physics, Chemistry & Materials/CompSci/Geology
2nd year: Maths, Physics A, Physics B
3rd year: Experimental and Theoretical Physics
4th year: Experimental and Theoretical Physics
As you can see, you can become a straight "physicist" in the second year. You could do some biology if you wanted, but it's definitely not compulsory. You can definitely get your fair share of physics and maths, and the DAMTP has an outstanding reputation
Here's a link to Cambridge's course structure:
http://www.cam.ac.uk/about/natscitripos/ps/structure/You definitely don't
have to do any biology; that's what's great about Cambridge's course - you don't have to marginalise yourself as a "chemist", "biologist" or "physicist". The sciences are linked, and the course lets you tailor your degree to your wants/needs.
I know that, but you could try reading the whole thread and you'll see that I corrected myself. It's not blacklisted but it's only "of limited use for arts subjects"; doesn't change the fact that it's completely useless for science at Cambridge, though