I dont know much about university level education.
Is it possible to get into a uni for, say, Physics, then change your mind and do a maths course?
My physics ability is much better than my maths ability, but I find maths more interesting.
It is possible, yes. Also, if you're basing your "ability" on how you're finding your A-levels and you're finding maths a challenge, you're about to experience a whole new level of anal gaping at uni. Especially at cam/imperial.
No. It's pretty difficult to switch courses. I don't know what year you're in but the deadline for Cambridge passed and the deadline for Imperial is in 5 days. Also, you should try actually getting into either one of those unis before considering switching courses.
Oh, and I don't think those courses accept gap years so if you're in your final year then good luck I doubt you'll make at either one for undergrad unless you can write a fucking sparkling personal statement in 5 days for Imperial.
Final year of HS.
I will get A* physics and A* maths, but in further maths I wont get more than an A.
I'm not too smart but I am one of the most motivated people I know, thats why I have okay grades - I do a lot of extracuricular work and recreational maths.
I have 3 other alevels but they arent related to my chosen field - history, art, english.