Whatever disadvantage from studying Std English or General Maths wouldn't really matter if you are willing to work hard for it. I had the same concern when I tried enrolling for BEc / LLB, the undergraduate dean said that it was merely preference, not requirement. But having said that, I would double check with the uni as this was 5 years ago.
Adv Eng is preferred because of the enormous amounts (and difficult) readings you will get within your law degree. There are a few units in your economics degree that would test your reading skills (political economy and history of economic thought) as you'll be reading primary sources of economists rather than secondary text book sources in those two units and it can get very confusing - espicially works from David Ricardo and Karl Marx.
Having Mathematics is an advantage because you would be using a fair amount of statistics and models in your economics degree. More specifically, within the units of macroeconomic analysis, microeconomic analysis, macroeconomic theory and policy, industry economics and policy, introduction to economic methods, economic modelling, applied econometrics (elective) and public finance.
Most of the units assumes very little knowledge of maths and the lecturers are very helpful in arranging extra lessons to help you get through the hard parts. I had a string of HDs in the areas statistics, regression analysis and advance econometric models having only studied General Maths.
