Hiya Zak!
If you genuinely don't enjoy engineering and don't think its for you - that is totally okay! I know so many people that retook year 12 for various reasons, and at my sixth form college, their situation was very similar to yours.
Repeating the year definitely won't discourage Universities from accepting you onto their courses, you'll find that a lot of students you meet will have gotten to were they are in a range of different ways - some may have taken gap years, gone back to education as a mature student, or like you, retaken a year and are starting university a year later than you would've done.
I can't speak much on your potential A-Level options and what they hold for you career or course-wise, as I study Journalism which is very far from being a STEM subject, but maybe have a look at some courses, speak with career advisers at your school, attend some UCAS Discovery events and open days of universities you're interested in, and speak with lecturers to get an idea.
I hope this helps, and best of luck in whatever you choose to do!
Thea (Kingston Rep)