Original post by FasHey there, economics student speaking.
In regards to career aspects, you can go into anything you want. Most Econ grads tend to move into the finance sector (so Investment Banking/Investment Management/Audit etc) or Consulting or Corporate Law. There is also the prospect of Economic Consulting at companies such as FTI Consulting, Deloitte, Capital Economics etc. Definitely research these career paths before embarking on them, but basically you can do anything really. Some grads also move into economist positions at places such as the Bank of England too.
In regards to what the course is like at University, it is very mathematical and theoretical. Its academically rigorous, so **** all relevant for careers in the real world, but really good for proving and explaining why we make the choices we make. It's a challenging course, by no means easy, so you have to keep on top of the work and you need to be smart (the top Universities usually ask for A*AA at A-Level and I haven't seen any Uni asking for anything less than AAB, and even then AAB is pretty rare)- and its usually compulsory to achieve an A at A-Level Mathematics (although if it is BA Economics that you go for, then Maths at A-Level isn't a necessity) - should also add that Economics is split up into BSc (Bachelor of Science) and BA (Bachelor of Arts) the BSc is far more mathematical and quantitative, whilst BA is more essay-based. However check the modules before choosing, because Economics at Cambridge and Durham are technically BA, but contain a lot of maths in them. *
A-Level Economics is very good for teaching you the fundamentals, but by no means necessary. At Uni you will be taught everything afresh, and the type of teaching and method of examination differ greatly from A-Level, however the actual content remains similar. You will learn Microeconomics and Macroeconomics, so learning the basics about Supply and Demand Analysis, Monopolies, Oligopolies etc. With Macro you'll focus on GDP/Unemployment/Balance of Payments and what not, learn about various Economic Models (such as Solow Growth Model, AK Model, Harrod-Domar Model) to capture macro variables.
A lot of the maths you do will revolve heavily around Algebra. In 1st year I had 2 pure Maths modules, the first one being a recap of A-Level Maths (Core 4 to be exact) and the second one revolved around Differential Equations and Difference Equations, whilst pretty much all the maths in 2nd Year has been about derivations and proofs. You'll need a strong grasp of Calculus (so Differentiation and Integration) as they tend to pop up quite frequently. And of course, Statistics. So Mean/Variance/Standard Deviation etc, will be present heavily throughout the degree, you'll most likely do pure stats modules in 1st year (which will cover S1,S2 and S3) and then Econometrics in 2nd year, which is a total mind****, but you don;t have to worry about that just yet.
Hopefully thats given you some insight, but if theres any doubts, just let me know.