clearly if you are putting 110% effort into your work and you are achieving fairly average grades you would be better deciding on a different path. I believe your school has done you a favor. Why not concentrate on something less academic, more people based, like anthropology? Religion?......
However, if you can't _obviously_ get into the top science set at your school, I'm afraid you have no chance at all of getting a place on a medicine course. I'm sorry to be harsh, but it is _that_ competitive. You would need to be in the top five students at a bare minimum. You'd probably need to be the absolute best student.
I hate to break this to you but if you are only reaching a 6A in ks3 where science is very simple your chances of progressing to a university as renowned as Imperial are pretty low. Perhaps you should stop complaining and look at this as a reality check, clearly if you are putting 110% effort into your work and you are achieving fairly average grades you would be better deciding on a different path. I believe your school has done you a favor. Why not concentrate on something less academic, more people based, like anthropology? Religion? They are subjects that require intelligence but of a different nature :)