Hi,
I do Maths, Further Maths, History & Economics at A Level and I'm currently in Year 12. I started off the year with the intention of applying for Economics at LSE, which was a factor in deciding my A Level choices. Honestly, I don't think it matters too much what your other A Level choice is as long as your maths side is strong- they want an A* for LSE. The guidance on the website on typical subject choices for straight Econ is 'Economics; Physics; History; Chemistry; English and Government and Politics, of which Economics, History, English and Government and Politics are good indicators of writing ability'. Also, for that course 3 subjects are sufficient so you don't have to take another subject for the sake of it.