First, definately finish uni! Its absolutely awesome at uni, and why anyone would want to leave prematurely is beyond me. When you've finished you have 45 years to do as much work as you want. Secondly, you do not go at a higher level, its just (iirc) if you achieve excellent marks on your police exams and are recognised as having a high potential, you can be recommended for the HPDS. Although, i've heard people that go through the HPDS are sometimes...er.. not as well recieved by colleagues than if they have worked their way up!
Also, i don't think having a degree gives you an edge for promotion either.