Definitely do Edexcel for Maths. The Edexcel textbooks are brilliant and you get a CD-Rom with them which has the exam cafe and a solution bank which shows you how to do every single question in the textbook! Edexcel also have a LOT of past papers for maths on their website. Try to get access to MathsNet and MyMaths if you can, since these are also extremely helpful! You'd have to do C1, C2, C3, C4 and two applied modules. For economics at uni, I'd recommend doing at least S1, and then either S2, M1 or D1 as your second applied module. S2 would probably be slightly harder than the other two however would probably help you more with Economics, since Mechanics is basically Physics and D1 is algorithms and linear programming, etc (and this module would be difficult to self-teach as it is all completely new concepts!)
Hope this helps, and Good Luck!
Lizzie