So what make a stronger application in terms of academics?
i) Good AS grades (a's and b's and more a's than b's)
ii) Good Subject choices (Maths, Economics, Further Maths (1 of 4 rather than 3) an essay subject, a science)
iii) Good Number of subjects (4-6 AS-levels, 3-5 A-levels)
And what makes a weaker application in terms of academics?
Not having the properties listed above.
(Special mention for Maths (mandatory at most strong unis) and Economics prefered at most unis and held by 80-90% of students at Top 5 unis))
In addition check out the generic thread I started on Soft and Hard A-level. That thread documents both uni blacklists and how many students at top unis have each A-level.
Here are some econ application specific points. Avoid
i) Overlapping subjects (e.g. Business and Economics)
ii) Less preferred (i.e. blacklisted) subjects e.g. ICT. see the LSE list. The Cambridge blacklist used to be here. But it is no longer there. So check out my thread for what the Cambridge preferences are.
Note if you don't have an essay based subject as one of your main A-levels, doing GS as an extra subjects is one way of showing that you can write an essay. But GS will only do you much good outside the Top 10.