Switching course at LSE

Hi, I have received an offer for Politics and Economics at LSE, starting in September. However, I am worried that studying politics might not be good for job prospects. Also, while I am very interested in studying politics, in the past few weeks I have realised I would prefer to study a degree that is very quantitative.
Therefore, other than selecting certain modules, I am wondering how easy it would be to switch courses to something like pure Economics while I am at LSE. I know you can switch either within the first few weeks of your first or second years. However, I do realise that switching to a course as popular as pure Econ may be hard. I am wondering if I manage to achieve an A* in Maths this summer will it improve my chances of switching in September, or whether it would be better if I focused on getting good grades in the Economics modules of first year before attempting to switch in second year.
(My other two A-levels are Chemistry and Politics, my school doesn't offer either Further Maths or Economics)