Should I get a maths or physics BSc?

Hi! So I'm graduating high school next year and I am certain that i want to get a doctorate in theoretical physics. Basically I love maths, and the only area of physics that truly interests me is the theoretical aspect and anything to do with the Universe (basically i love all maths but not all physics). So... show more Hi! So I'm graduating high school next year and I am certain that i want to get a doctorate in theoretical physics. Basically I love maths, and the only area of physics that truly interests me is the theoretical aspect and anything to do with the Universe (basically i love all maths but not all physics). So I'm torn between getting a BSc in maths and then moving onto theoretical physics or if its necessary for me to have a BSc degree in physics in order to be able to then move on to theoretical physics. So i guess my question is, would it be possible/recommended for me to start with a BSc in maths to then go on to theoretical physics or should i stick to physics from the start or is there another option? (Also I think in the UK which is where i'll study there's no such thing as getting a major and a minor so I don't think i can study two courses at once but correct me in i'm wrong).
(If it helps I'm aiming for either Oxford, Cambridge or King's College London whichever I actually manage to get into haha)
Thanks!
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