i don't know what course to do at university?

I had the same problem, I didn't know what I wanted to do but knew I wanted to continue in education. You're right in trying to pin this down now as It's best to know what you want to apply for so you can tailor you statement what with the number of university applicants increasing in the UK.

I couldn't chose just on personal reasons alone, as I was equally interested in 2 subjects and wouldn't of minded studying either at university. In the end I looked at all the courses I could do and what universities had a good reputation in these fields. It basically came down to statistics in the end one area had a much higher employment rate after graduation then the other in general then I just looked at which places looked the most interesting in terms of the course and the towns.

Try looking at what you can do and where you can do it and make a list, then compare the university's, locations and check out post graduate employment rates for each field, the ratio of male and female student etc. Your be there for 3 years (if its an undergrad course so its worth considering not just the school but the "life style" available.

Basically If you can't think about the specific course right now it might help to think about the other factors, it may help you to at the least rule out some options.

All 4 years of high school affect University, but colleges don't really look that much into electives! They focus more on academics! It really depends on where you want to go! Most of the universities I applied to this year only counted the academic courses in my GPA anyways. So my answer is no, the elective won't affect your chances! There are far more important things for them to consider!