Don't bother enrolling in your entire 3 years now, it's completely unnecessary and will just result in you changing subjects this time next year (seriously). Putting in 2nd/3rd year subjects now, as opposed to next year makes no difference. First year science is usually fairly generic (cough two semesters of chemistry and biology cough).
You can add subjects to your study plan from home, it doesn't have to be at uni or even in January. You're free to make an appointment with your student center, but at this time of year it's busy due to the influx of several thousand new students. In terms of what you'll miss...not much uni stuff. The OP is a summary of what happens, but it's your first chance to meet students in the course etc. and that's probably more important than picking your subjects
@mike
The screen one is only if you want to write scripts, there's not much focus on "creative" writing (ie prose). I did ideas and practice. I also don't really like poetry but it was handled well (modern poems as well as ye olde english ones) and I enjoyed writing my piece.