I would not take A-Level Chemistry, as it is the most work, and the same goes for A-Level Biology. However, remember A-Level Physics has 25% Mechanics topics. So if you do A-Level Maths, A-Level Physics, A-Level Further Maths, it will be like doing 1.5 A-Levels, since topics overlap and there's no tedious long essays in them. Also, A-Level Economics essays are not difficult, nor is it that quantitative like it is at degree level.
If you do A-Level Further Maths, you will do Further Mechanics, Further Decision, Further Statistics or Further Pure. It would be beneficial to do Mechanics in A-Level Further Maths, as it will help you with A-Level Physics immensely so.
If you want to apply to Cambridge and LSE for Economics, then you will need to take A-Level Maths, A-Level Further Maths, A-Level Economics and A-Level Physics too.