For an economics degree at a top uni such as Cambridge or LSE, you will practically need FM if your school offers it. Regarding the physics side of maths, I am assuming you mean mechanics? You can always take FM, ask your teachers what modules your school does for further maths, and if it is largely mechanics based, and you really find it bad, drop it afterwards. Chemistry isn’t too bad in year 12 at least, a lot of marks are calculation based, and if you spend time properly understanding the content, and doing past paper questions, you will be fine. If I were you, I would take Chemistry and Further Maths, as your 3rd and 4th options, assuming you are taking another subject, and you can drop either at any point depending on how you find it.