I'm currently doing maths, economics, geography and Spanish and I'm thinking of studying economics/economics with … (another subject eg Spanish)
I would say definitely take maths and if you believe you are good enough further maths too if you are looking LSE.
I'm finding Geography is actually a really good subject with economics, it links really well.
History would also be another good subject to do as it is essay based and shows that you can write essays well.