Most unis don't want to set students up for failure. If they are willing to let you in with the qualifications you have, they must think you have a chance of passing the course.
If you struggle with exams, try to find a course which is at least partially coursework based, as this will take the pressure off a bit. You can also spend a lot of time in first year working on your exam technique- as the marks probably won't count, or will only be a tiny percentage of your final grade.
It might also be worth discussing your concerns with someone at open days- they may be able to let you know how other students with BTECs coped with the course.