If you pick Further Maths, they won't take that as an A Level entry requirement for straight Economics and they'll put you onto the application for Economics and Mathematics.
So, unless you want to do Econ and Maths and not straight Econ, don't pick further maths- unless it is your fourth choice A2. Call them up and check that, it may be different.
I suggest Mathematics, Economics and anything else from Philosophy, English, a science, anthropology, something in the arts.
See what options you have at the place you're studying, even a language would be great.
Where else are you applying? I mean, the best Economics institutions alongside LSE are Oxbridge, Warwick and UCL too, and they pretty much all are attracted to the same A2s.