Its hard to keep your options open for these two subjects - for a chemistry degree at a top university, they you would likely be disadvantaged by not having another science at a level. For economics on the other hand, especially at Cambridge/LSE/UCL/Warwick, further maths would be highly beneficial. Although a level economics isn't essential for an economics degree, if your school offers it as a subject you may leave admissions tutors wondering why you passed up on the opportunity to study it at a level, but then want to read it at university.
So by doing maths, further maths, physics and chemistry you would be in a great position to do chemistry at a top university (depending on your grades ofc), and would be in a good but not ideal position for economics.
Alternatively, maths, further maths, chemistry and economics would certainly be good for economics, but i'm not sure if that would suit chemistry (not qualified to say)
If you are prepared to do 5 then you could do maths, fm, chem, phys, econ, which would be ideal for both subjects.