As is the case with most others in the thread, I'm an American. I got a 4 year degree from what I always heard called a "top 10" CS program. I took some programming classes in high school, and taught myself some things before I started school, but that is definitely not what will make or break your education. What no one has mentioned (at least outright) is that University CS is not intended to teach you to program. I don't know what a similar school would be in the UK (maybe College?), but in the US trade schools/technical institutes are where one would go to learn to program. You go to University to learn to think, no matter what your area of study is. I'm sure for "Computer Graphics II" you will have to program, but they're not going to teach you how to implement a tree structure in C++ or something. They're going to teach you what data structures and algorithms are important to the problem set, and how to think about breaking down relevant problems and solving them.
To summarize, any work you're doing now on learning how to program is definitely putting you ahead of the curve in terms of the "grunt work". What might be more important to you if you want to get ahead on what they will actually be teaching is reading a book on algorithms or discrete math. The discrete math will definitely be more helpful to you in a "Functional Programming II" course than bettering your C skills, etc.
With all of that said, and more important than any of it, you are motivated and interested in what you intend to study at University. That's going to serve you better than any amount of programming practice, reading, etc. ahead of time.