If you're thiinking about economics as a university degree then I would strongly suggest taking Maths to broaden the uni's you can apply to (as most of them require A level maths to do it!). However you can still get offers for Economics without having taken A level maths - I have 4 out of my 5 offers for economics without maths A level.
As stated above, maths would also go really well with chemistry and biology in the science-y department side of your subjects. Politics would only really help with economics, and then again only really in the A2 year (assuming you take both to A2 like I did), but I found politics fun and some of the things you learn in the second year are a bit mindboggling and bamboozling.
On the other hand, just do something that you enjoy or like the sound of! If the worst comes to the worst, you can always change the subject normally after two weeks (I did this with Geography where I was getting E's in homeworks etc., changed it to Chemistry and got an E overall in my AS level) - I must stress though that I was never really motivated to study chemistry, so if you do a subject which you have a real passion for then you will be motivated to study it in your own time and therefore stand a better chance at doing well in it.