Nothing's ever guaranteed - I personally know of someone who got straight 9s in GCSEs and had quadruple A* predicted for A Level not even be invited for an interview at Oxford (sorry if it's heresy to mention the other one in this forum). Life's crazy like that sometimes. However, I must say your odds are looking good so I'd start prepping a bit now. But take it slowly. You don't want your answers to sound rehearsed, you want to learn to think out loud, and you don't want to come off as cocky!