A/A* in Maths GCSE essential: Bath, and several others.
For an Economics degree:
A/A* in Maths GCSE "practically essential": all the top 10.
90%+ in Maths AS "practically essential": Cambridge, and possibly Oxford/LSE if you give in your UMS marks.
A in Maths AS essential: all the top 10, although the odd one might be "practically essential".
A* in Maths A-level essential: Cambridge, Oxford, LSE, UCL
A in Maths A-level essential: all the top 10.
Doing both Business and Economics A-levels "strongly discouraged": all the top 10.
Super mathematical courses: Cambridge, LSE, UCL, Warwick, Oxford (http://i50.tinypic.com/wrf8mx.png
as a taster)
So you need to take Maths A-level, you should take Further Maths A-level if considering a top 5-10, and you should take Economics if possible, although it's not a big deal if you don't