I am during the process of writing my ps. I am applying everywhere for physics except cambridge where i am puuting down Natural sciences as there is no seperate physics course.

My question is whether it is ok if i write only about physics, or should i mention chemistry even if it is just one sentence, because the NatSci in cambridge is a mixture of all three sciences?

If you're mostly interested in physics, then write about phyiscs, but it's worth mentioning chemistry as well.

Cambridge should take into account that other universities will just be taking candidates for "straight" physics. Or you could apply to Oxford