Can i go to university without finished high school? I wasn't sure if this would be off-topic or not. 

Basically i have half finished high school here in Slovenia, it's slightly different here, i went to 3+2 program. I have finished 3 years but i don't wish to continue the extra 2 years, especially because they're not teaching me anything, in 5th year they teach about programming stuff i knew when i was a first year. And the extra two years would earn me a diploma for finished high school, not sure how the 3 years are called in English. So instead of wasting 2 years i thought if i could even apply for University in UK? I have a friend who lives in UK which would make it easier to move, but my question is if i can even apply? I've seen undergraduate courses and the requirements, but nothing there can apply to me? Or at least i'm not sure, is there some kind of exam that i can take or something similar?

Thanks in advance!
I'm sorry but no, you can't. Every respectable university requires a high-school diploma (or equivalent degree) for students to enroll. Exceptions are extremely rare and apply only to highly gifted students in certain subjects.

However, you can self-study in your free time with more advanced textbooks and online courses. If your home-town has a university, you may be able to attend some lectures as a guest, without being formally enrolled.