I did NatSci at Cambridge and believe me - you will be at a serious disadvantage if you don't do it, regardless of what you currently think you want to specialise in. The NatSci course is specifically designed to be broad at the beginning and 'funnel' towards a speciality later. You're broadly either Physical (Physics/Chemistry/Maths) or Biological in your Part 1A (=first year), but Chemistry is highly relevant to both of these strands.