Maths + physics applicant here (Peterhouse), my PS is half half almost between maths and physics, but I end with a stronger note on maths, (from a philosophy of maths angle). Anyways, I'm myself most interested in theoretical HEP (QFT/string theory) type. Cambridge maths course as others say is very applied (you can pretty much do all theo physics by yr 3), I found the maths side of physics (ie foundations and derivations) much more interesting than practicals, so I opted for maths + physics. A stronger maths background would aid your reasoning skills for future physics career (more so than a practical lab/pure physics route). Unless there's a huge preference of doing lab work, I will say maths + physics at Cambridge is way superior than natsci in future physics research type career, (mind you it's only a first year course).