^ Studying CS in O and A levels might not help you much (I don't know about A level Computing, but I know the O level stuff is thoroughly useless). The point I was trying to make is, that you're willing to choose a career based on complete ignorance other than some misguided surveys on career related earnings. University is not like O/A levels, where you take a subject and if it doesn't interest you, you can drop it, or just give the exam and not worry about it any more. University is considerably harder, more demanding, and if you choose a bachelors program on some whim, you will have a terrible time if you find out you hate it.
Do the following:
- Start learning one compiled language (C++) and one scripting language (Python).
- Start a personal programming project with a reasonably large scope (for example, a healthcare system which keeps track of patient records, or something like that). Try to implement it and you'll find out what REAL programming is like.
- Visit the websites of universities you would be interested in applying to, and check out the required course descriptions for their CS program to get an idea of what you would be getting in to.
To answer your last question: Usually CS programs start from the very basics. However, chances are that more or less all of your peers will have some to significant programming experience, and you will be at a SERIOUS disadvantage if you don't know anything. University moves fast (especially the semester system if you'll be going abroad), and you will suffer greatly in the beginning at least.
As far as your obsession with career earnings is concerned. I know of a guy who has been approached by Google multiple times asking if he would work for them, and they offered him a salary in the $150-200K a year range. He turned them down every time saying Google isn't worth his time and that he can earn more on his own. He offers private consulting services related to CS, and charges $200/HOUR. Right now he is doing his PhD at the University of Waterloo. Does that make you think twice before you take those MSN/Forbes surveys seriously?
It doesn't really matter what you do. What is more important is how passionate and good you are at what you are doing. Money is just a by-product of it all. You probably won't find many examples of people who earn a lot but hate what they do.