I would safe to say that if you find a course that you are interested in and passionate about, you are more likely to get better grades than someone who is doing a degree because they were unsure of what to do. Like someone has already said, it's all about how much you put in.
Secondly, I got a mixture of grades at GCSE and A level, from 5 A's down to an E at GCSE and BC and D* at A level. To say the least, by the time I was starting A Levels, I wanted to be studying teaching, not maths or english. It didn't interest me.
Uni has so many teaching methods too. I have field trips and lecturers who literally bounce off the walls, but all of it is working towards my long standing dream of teaching. That's what motivates me. Be motivated and you'll get there. I got lower grades than my uni wanted, but they still accepted me. It really depends what uni and if they can see you as a student. Good luck. And remember that uni isn't the only way, so don't be disheartened.