How to get a phd in astrophysics in Cambridge and Princeton? Pleeeeeeaasseee answer :(?

I am still in middle school (which gives me time to think and plan my future before i slack of at high school). I don't live in the uk nor the us. I really want to get a phd in astrophysics someday. I know it takes really long, but I heard that Cambridge has a program which allows me to get a bachelor's and master's degree in 4 years. But the phd program is quite long, 4 years? (I think?) but I also heard that in the us, it takes an average of 2 years. Princeton has always been my dream since I was 10. Originally, I wanted to go to Princeton for my undergraduate's and graduate's degree. Can I go to Cambridge for my bachelor's and master's in physics and Princeton for my phd in astrophysics? Oh yeah, and what certificates do I have to get? I know how the American sat's work and know an international school 30 minutes away that has the sat's ( I can't transfer there cause it's too expensive). And even if i do the sat's, do I need to go to ap classes? I heard that you can take ib as well, and there are lots of schools here which offer that, but mostly they're international schools and they're expensive and the studies aren't that good either. Oh yeah and my mum is ok with letting me study abroad for my sixth form.