Yo I was at the same position as you last year. I had got BBCD at AS, only for them to predict me BDE after my performance in my first string of mock exams. With a little one-to-one sit down with my subject teachers explaining what I will try to achieve in the coming months leading to exams in order to sit up, focus more and mess about less, I managed to get my predicted up to BBB, picking up offers from ABB courses from Universities such as UEA, Kent & Leicester.
Just explain to them in a nice but passionate manner that you aspire to go to a Uni asking for ABB and would go out of your comfort zone in order to achieve this goal. But at the same time actually take college seriously after (something I didn't and consequently ended up at Essex), because if they were to leave your grades as they were and it turned out you got something like AAB, you can still go through adjustment and apply for the specific courses at the ABB unis.